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Crop Protection 25 (2006) 592–603

www.elsevier.com/locate/cropro

Effects of liquid insecticide baits on Argentine ants inCalifornia’s coastal vineyards

Kent M. Daanea,�, Karen R. Simea, Brian N. Hogga, Mary L. Bianchib, Monica L. Coopera,Michael K. Rustc, John H. Klotzc

aDivision of Insect Biology, Center for Biological Control, University of California, Berkeley, CA 94720-3114, USAbUniversity of California Cooperative Extension, San Luis Obispo County, 2156 Sierra Way, San Luis Obispo, CA 93401, USA

cDepartment of Entomology, University of California, Riverside, CA 92521, USA

Received 6 May 2005; received in revised form 10 August 2005; accepted 12 August 2005

Abstract

Liquid ant baits were evaluated for control of Argentine ants, Linepithema humile (Mayr), and associated mealybug pests

(Pseudococcus species) in commercial vineyards. In all trials, liquid baits were an insecticide dissolved in 25% sugar water. In 2000, two

liquid baits—crystalline boric acid and imidacloprid—were deployed in ground-based dispensers at rates of 85 (site 1) and 175 (site 2)

dispensers per ha. Season-long ant densities were significantly lower than a no-insecticide control in only the boric acid treatments, and at

only 1 of 2 sites. In 2001, four liquid baits—imidacloprid, fipronil, and thiamethoxam, each mixed at 0.0001% (A.I.)—were delivered in

ground-based dispensers at a rate of 120 dispensers per ha. There was no treatment impact on ant or mealybug densities. In 2002, a liquid

bait—thiamethoxam, mixed at 0.0001% (A.I.)—was delivered in ground-based (site 1) and canopy-based (site 2 and 3) dispensers that

were recharged every 2wk and cleaned every 4wk, and deployed at rates of 160 (sites 1 and 2) and 620 (site 3) dispensers per ha. There

was a significant reduction of season-long ant densities in liquid bait treatments at all sites, and of mealybug densities at 2 of 3 sites; crop

damage was significantly lower in the liquid bait treatment at all sites. The results are discussed with respect to the methodologies used to

deploy liquid baits and the development of a viable commercial program.

r 2005 Elsevier Ltd. All rights reserved.

Keywords: Argentine ant; Linepithema humile; Pseudococcus; Liquid baits; Vineyards

1. Introduction

The Argentine ant, Linepithema humile (Mayr) (Hyme-noptera: Formicidae), is an invasive species that hasestablished over a wide geographic range, often withdamaging economic and ecological impacts (Holwayet al., 2002). Most commonly recognized as urban pests(Knight and Rust, 1990a; Rust and Knight, 1990), theseants also have damaging impacts on natural biota and inagricultural production. The Argentine ant’s unicolonialnest structure, high population density, and efficient use ofresources provide an advantage in interspecific competition(Human and Gordon, 1996; Holway, 1998; Chen and

e front matter r 2005 Elsevier Ltd. All rights reserved.

opro.2005.08.015

ing author. Tel.: +1559 646 6522; fax: +1 559 646 6593.

ess: daane@uckac.edu (K.M. Daane).

Nonacs, 2000). As a result, Argentine ants displace nativeants and other invertebrate and vertebrate species (Ward,1987; Sanders et al., 2001; Suarez et al., 2002).In agricultural systems, Argentine ants are associated

with outbreaks of phloem-feeding homopterans, fromwhich they collect honeydew and, in return, protectthem from natural enemies (Way, 1963; Barzman andDaane, 2001). In California’s coastal vineyards, theArgentine ant has been implicated in outbreaks of thegrape mealybug, Pseudococcus maritimus (Ehrhorn), andthe obscure mealybug, Pseudococcus viburni (Signoret)(Hemiptera: Pseudococcidae) (Phillips and Sherk, 1991).These mealybugs damage the crop directly by accumu-lating in the grape clusters, and indirectly by transmittinggrapevine closteroviruses and by secreting honeydew,which fouls the crop (Flaherty et al., 1992). Exclusion

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experiments, using sticky barriers on the vine trunk, haveshown that removal of Argentine ants lowers bothmealybug densities and crop damage (Daane et al.,unpublished data).

Ant controls often rely on contact insecticides that areused as barrier treatments (Rust, 2001; Klotz et al., 2002,2003). These chemical sprays provide only partial antcontrol because they kill or repel foragers but have littleimpact on the queens (Knight and Rust, 1990b; Rust et al.,1996). Foragers constitute only a small fraction of theworker force and are quickly replaced by nest mates thatreach maturity during the treatment period. Additionally,degradation of these chemicals commonly occurs within 30days of application, negating any residual effects (Rust etal., 1996) and increasing the need for reapplication.Granular treatments for ant control are commerciallyavailable for use in agricultural systems, especially nurseryoperations (Hedges, 1997; Costa et al., 2001). Solid baits,typically targeting protein-feeding ants, have been some-what successful (Hooper et al., 1998; Taniguchi et al., 2005;Tollerup et al., 2005). However, many pest species,including the Argentine ant, primarily forage for sugars(Klotz et al., 2002), which necessitates the development ofliquid baits (Silverman and Roulston, 2001; Rust et al.,2004). For these reasons, more effective and environmen-tally sound ant control practices are needed for vineyards,especially for managers developing sustainable farmingpractices.

Our initial goal was to compare the effectiveness ofvarious insecticide materials for Argentine ant control invineyards, especially those applied as liquid, sugar-basedbaits. The project goals changed as we discovered that theliquid bait delivery techniques adopted from urbanprograms (Klotz et al., 2002) needed to be adjusted forlarge-scale agricultural use. The field trials presented herespanned 3 years (2000–2002) and were conducted inCalifornia’s Central Coast (2000 only) and North Coastwine grape regions. In the first year, we compared theeffectiveness of a standard barrier spray (chlorpyrifos) andtwo liquid baits (boric acid and imidacloprid, delivered in25% sugar solutions). Thereafter we focused on liquidbaits, rather than a barrier spray, for several reasons. First,liquid baits have provided Argentine ant control in bothurban settings and citrus orchards (Klotz et al., 2002, 2003;Tollerup et al., 2005). Second, insecticides applied as abarrier kill or act as a repellant and consequently onlyaffect foragers whereas baits are returned to the colony.Third, baits minimize undesirable environmental impacts:only a small amount of insecticide is placed in the field, theinsecticide is contained rather than applied to the crop orsoil, and the bait dispensers can be designed to minimizebait delivery to non-target insects (e.g., pollinators)(Taniguchi et al., 2005). We evaluated treatment effectson mealybug densities and crop damage, as well as on antdensities. Our results are discussed with respect to thepotential commercial development of liquid ant baits invineyards.

2. Materials and methods

2.1. Field sites

Bait trials were conducted at commercial vineyards inthe Central Coast region (Edna Valley appellation in SanLuis Obispo County and Santa Maria appellation in SantaBarbara County) and the North Coast region (Carnerosand Dry Creek appellations in Sonoma County). Allvineyards were mature (48 year old), managed for wineproduction, on drip-line irrigation systems, and cleancultivated by running a disc over the row middles and/orapplying an herbicide (glyphosate) to the berms. Duringeach trial period, no additional insecticide applicationswere planned, but sterol inhibitors and/or sulfur wereapplied to control powdery mildew (Uncinula necator

Burrill). With one exception (Dry Creek appellation), allvineyard sites had histories of mealybug and Argentine antpest problems. The Central Coast vineyards were infestedwith the obscure mealybug and the North Coast vineyardswere infested with the grape mealybug. Plot size and thenumber of insect samples collected varied among trials andare therefore described separately for each experiment. Notrials were conducted using the same treatment plots usedin a previous season.

2.2. Insecticide application

The barrier material tested was chlorpyrifos (Lorsban-4E, Dow AgroSciences, Indianapolis, IN), which wasapplied to the lower vine trunk and 0.3m diameter areaof the ground around the trunk-base using a hand-heldspray applicator at the label rate of 2.35 L (A.I.) per ha.The liquid bait materials and application rates used were0.5% crystalline boric acid (Sigma Chemical, St. Louis,MO; 99% [A.I.]), 0.05% (in 2000 only) or 0.0001%imidacloprid (Bayer CropScience, Kansas City, MO),0.0001% fipronil (Rhone-Poulenc, Research Triangle Park,NC), and 0.0001% thiamethoxam (Syngenta Crop Protec-tion, Richmond, CA). All of the percentages are weight tovolume. Each chemical was technical grade material, withthe exception of the 2000 Central Coast trial, in whichimidacloprid was delivered as Admires (BayerCropScience, Kansas City, MO).In all liquid bait treatments, insecticides were dissolved

in 25% sugar water and delivered in bait dispensers. Whenwe were mixing fipronil and imidacloprid technical, we firstdissolved the material in 1–2ml of acetone and then mixedinto the 25% sugar water because these materials are notvery water soluble. The dispenser design changed duringthe study, incorporating improvements developed throughobservations of ant behavior and dispenser maintenanceand labor costs. In 2000, the dispensers used in the CentralCoast region were constructed from 10� 5.5� 7 cm plasticstorage containers, hereafter referred to as rectangulardispensers. Six holes (0.5 cm diam) were drilled in the lidsfor entry, and the interior was filled with shredded aspen

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wood (Aspen Snow Cool, El Cajon, CA) to provide asurface for the ants to walk on. The rectangular dispensershad a 500ml capacity and were filled with E350ml ofliquid bait. A different design was used in the 2000 and2001 North Coast trials to make use of materials readilyavailable to growers and to develop a dispenser with alarger storage capacity. The dispensers were constructedfrom 0.6m sections of PVC pipe (0.6 cm diam, PVCSchedule 40 Pipe). Three holes (1.27 cm diam) were drilledalong a line, and the dispensers were filled with shreddedaspen wood and capped. The 0.6m PVC dispensers had a1725ml capacity and were filled with E1400ml of liquidbait. In 2002, the PVC design was modified by usingshorter sections (0.3m), drilling a single hole (2 cmdiam) inthe center of the pipe, and using a piece of plastic swamp-cooler pad (30� 6 cm) instead of wood for the internalstructure. The 0.3m PVC dispensers had a 905ml capacityand were filled with E600ml of bait solution. Therectangular and PVC dispensers were deployed on theground underneath the vines, typically near the trunk; theywere partially buried in the soil (about 5 cm) to moderatetemperature and light exposure and to provide better trailconditions for foraging ants. In 2002, we also tested adispenser that could be placed on the vine, rather than onthe ground, which was based on designs of monitor tubesfor ant feeding activity (see Klotz et al., 1998). Thedispensers were made from 250ml polypropylene centri-fuge tubes (Corning Inc., New York, NY). A 2 cm hole wasdrilled in the cap and squares of Weedblock (EasyGardener Inc., Waco, TX) were placed between the cap

Table 1

Changes in experimental design and locations during the 3-year period

Year Appellationa Treatment

2000 Edna Valley Boric acid/bait

(Central Coast) Imidacloprid /bait

Chlorpyrifos

No insecticide

Santa Maria Boric acid/bait

(Central Coast) Imidacloprid/bait

Chlorpyrifos

No insecticide

Carneros Boric acid/bait

(North Coast) Imidacloprid /bait

Chlorpyrifos

No insecticide

2001 Carneros Imidacloprid/bait

(North Coast) Fipronil/bait

Thiamethoxam/bait

No insecticide

2002 Carneros Thiamethoxam/bait

(North Coast) No insecticide

2002 Carneros Thiamethoxam/bait

(North Coast) No insecticide

2002 Dry Creek Thiamethoxam/bait

(North Coast) No insecticide

aGrape growing region.

and the filled tube. Weedblock, a permeable plastic mesh,minimized leakage when the tube was inverted, but wascoarse enough to allow ants to remove the liquid oncontact. The tubes were filled to E200ml with baitsolution, inverted, and tied to the vine trunk near thecordon or crown. The type and number of dispensers usedper plot is described for each experiment, and summarizedin Table 1.

2.3. 2000 trials

Our initial trials sought to compare liquid baits (boricacid and imidacloprid) with the standard barrier spray(chlorpyrifos) and a no-insecticide control. Trials wereconducted in two Central Coast vineyards (Edna Valleyand Santa Maria appellations) and one North Coastvineyard (Carneros appellation). In each, treatments wereset up in a randomized complete block design, with four(Central Coast) or six (North Coast) replicates. Eachtreatment plot was 0.1 ha (5 rows by 25 vines), with bufferrows and vines between each treatment. To reduce theimpact of any pre-treatment variability in ant populations,visual ant counts were taken on four randomly selectedvines in each treatment plot. For these visual counts, asection of the trunk or cordon—with ant activity—wasselected and the number of ants moving across that sectionwas recorded during a 60 s observation period. All plotswere blocked to balance ant density, and the fourtreatments were then randomly assigned within blocks.

Application method Dispensers per ha

Rectangular dispenser 175

Rectangular dispenser 175

Barrier spray —

— —

Rectangular dispenser 175

Rectangular dispenser 175

Barrier spray —

— —

0.6m PVC dispenser 85

0.6m PVC dispenser 85

Barrier spray —

— —

0.6m PVC dispenser 120

0.6m PVC dispenser 120

0.6m PVC dispenser 120

— —

0.3m PVC dispenser 160

— —

250ml dispenser 160

— —

250ml dispenser 620

— —

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At the Central Coast sites, chlorpyrifos was applied on29 May (Edna Valley appellation) and 26–27 May (SantaMaria appellation). Rectangular dispensers for the boricacid and imidacloprid treatments were deployed on 16–17June (Edna Valley), and 10–11 July (Santa Maria).Dispensers were deployed in every row, and spaced at4–5 vine intervals within each row (E175 per ha, based on1480 vines per ha); the dispensers were staggered inneighboring rows so that they were not adjacent to eachother across rows. At the North Coast site, chlorpyrifoswas applied on 16 May, and the 0.6m PVC dispensers werefield deployed on 3 June, placed in every row, spaced at 10vine intervals within each row (E85 per ha, based on 1480vines per ha), and staggered in neighboring rows. At theCentral Coast sites, the dispensers were refilled as needed,about every 6wk; at the North Coast sites, where the larger0.6m PVC dispensers were used, the liquid bait was notrefilled or replenished during the trial period.

To compare treatment effects, visual counts were madeto measure ant density, as described previously. At theCentral Coast sites, ants were counted (60 s per vine) on 8vines per plot (32 vines per treatment per sample date),every 2wk from 7 July until harvest. Mealybug densitiesand crop damage ratings were not taken at the CentralCoast sites. At the North Coast sites, visual ant counts (30 sper vine) were made on 6 vines per plot, every 2–4wk from15 June until 16 November (36 vines per treatment persample date). On the same sampling date, mealybug countswere made on 4 vines per plot (24 vines per treatment persample date), using a sampling technique developed byGeiger and Daane (2001). In brief, we searched each vinefor 3min, recording the numbers of settled immature(second and third instars) and adult mealybugs andmealybug ovisacs. An experienced sampler could determinethat part of vine where mealybugs were most likely to occurat a given time of year, which allowed adjustment forseasonal changes in the distribution of the mealybugpopulation on the vine. Bark on the trunk, cordon, andspurs was stripped away when necessary. To assesstreatment impact on crop damage at the North Coast site,we rated grape clusters at harvest on a 0–3 scale: 0 ¼ nomealybugs, 1 ¼ honeydew and/or o10 mealybugs present,2X10 mealybugs, sooty mold and honeydew present and3 ¼ a heavily infested, unmarketable cluster (see Geigerand Daane, 2001). Clusters in direct contact with woodyparts of the vine were preferentially sampled, whenavailable, because such clusters are more likely to beinfested with mealybugs (Geiger et al., 2001). Crop damageratings were taken from 3 vines per treatment plot, with 3clusters rated per vine (54 clusters per treatment). For ant,mealybug, and cluster samples, vines were randomlyselected from the middle 2–3 rows of each plot.

2.4. 2001 trial

We compared three bait-delivered toxicants (imidaclo-prid, fipronil, and thiamethoxam), deployed in the 0.6m

PVC dispensers, and a no-insecticide control. The trial wasconducted in a 16 ha Pinot Noir block in the North Coastregion (Carneros appellation). The treatments were set upin a randomized complete block design, with four replicatesper treatment. Each treatment plot was 0.17 ha (10rows� 25 vines), with 10 vines between each treatmentplot, and 3 rows between each treatment block. Thedispensers, which were in the field from 11 July to the endof the trial in October, were placed in every row, spacedevery 5–10 vines within each row (E120 per ha, based on1480 vines per ha), and staggered between neighboringrows. They were refilled once (15 August).Ants were counted visually (30 s per vine), as described

previously, on 6 vines per plot with counts taken every2–4wk from 1 February until 16 October (24 vines pertreatment per sample date). Indirect measurements of antdensity were based on feeding activity, which was assessedas the amount of sugar water removed from 50mlpolypropylene centrifuge tubes (Corning Inc., New York,NY) tied to the vine trunk (see Klotz et al., 1998). Similarto the 250ml dispenser design, the monitoring tubes hadholes drilled in the caps and were fitted with squares ofWeedblock that held back the sugar water when the tubeswere inverted. The monitoring tubes were filled to 45mlwith 25% sugar water, weighed, and placed inverted on thevine trunk for 24–48 h (depending on ant activity). Theywere then brought back to the laboratory and reweighed.In this manner, we measured ant feeding activity every1–2wk from 29 June until 26 October, with 16 monitoringtubes per plot (64 tubes per treatment per sample date). Oneach sampling date, two additional monitoring tubes perplot were attached to ant-excluded wooden stakes todetermine the amount of sugar water lost to evaporation,which was averaged across all plots and used to adjust thefinal weight.Mealybug densities were evaluated on 4 vines per plot,

every 4–5wk from 1 February until 16 October (16 vinesper treatment per sample date), using the methodspreviously described. Similarly, crop damage ratings weretaken at harvest-time, as previously described, from 5 vinesper plot, with 3 clusters per vine rated (60 clusters pertreatment). For all samples, vines were randomly selectedfrom the middle 6 rows of each plot.

2.5. 2002 trials

We conducted three experiments in 2002 that, based onthe results of the 2001 trial, incorporated several changes inplot design and dispenser design and deployment. In 2000and 2001, the blocks were arranged to minimize the acreagethat grower–collaborators left untreated, and to block forpre-treatment ant densities. This resulted in differenttreatment plots in the same rows, separated by buffervines. We observed that plot separation was more difficultto achieve along, as opposed to between, vine rows becausethe ants tended to move along the trellis wires andirrigation tubes that run the length of each row. For this

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reason, in 2002 we established every plot in a separate setof vine rows, but still employed a randomized block designwith blocking across row sections. Improvements indispenser design included the use of both 0.3m PVC and250ml tube dispensers, described previously. We refilledthe PVC dispensers every 2wk and replaced them entirelyevery 4wk to minimize fermentation and bacterial growth.The 250ml dispensers were cleaned and replaced every 2–3weeks. We also deployed the dispensers earlier in theseason and increased the density of dispensers per ha.

To reduce any site specific effect of ant and mealybugpopulations, vineyard management practices, or vineyardlocation, multiple sites were used. A 10.7 ha Pinot Noir cvblock (Carneros appellation) was split into two trial sites.In the first site, a 3.2 ha block (80 rows by 60 vines) wasestablished with two treatments—thiamethoxam in liquidbait and a no-insecticide control—set in a randomizedcomplete block design with four replicates. Each treatmentplot was 0.18 ha (5 rows by 50 vines), with a 5 row bufferbetween each plot and block. The bait solution wasdeployed in the 0.3m PVC dispensers, which were placedin every row, spaced every 5–10 vines within each row(E160 per ha, based on 1480 vines per ha), and staggered inneighboring rows. They were deployed from 28 March to10 October. In the second site, the experimental design wasthe same with the following exceptions: the bait wasdelivered in the 250ml dispensers, deployed on 13 May(also at a rate of E160 per ha), and the experimental blockwas smaller, (80 rows by 40 vines) with each plot 0.12 ha (5rows by 30 vines).

The third trial was conducted in a 16.1 ha vineyard (DryCreek appellation) with mixed Zinfandel cv and PetiteSyrah cv blocks (3–7 ha each); all blocks were contiguousand managed similarly. Because this vineyard had onlyrecently (within the previous year) been invaded byArgentine ants, both ant and mealybug densities werelow relative to the other trial sites. We established threeblocks, each consisting of two 0.05 ha treatment plots (5rows by 10 vines) that were separated by 12–18 rows. Thetwo treatments were the same as the Carneros sites:thiamethoxam in liquid bait and a no-insecticide control.Because the mealybug density was low, which mightincrease movement of ants in search of an alternative foodsource, we wanted greater separation of treatment plots toreduce recruitment of ants from the control plots into thethiamethoxam bait plots. For this reason, there were 25–50rows separating each block. The liquid bait was deliveredin the 250ml dispensers, deployed on 27 June in every row,placed on every other vine within each row (E620 per ha,based on 1235 vines per ha), and staggered in neighboringrows.

At all sites, visual ant counts (30 s per vine) were madeon 10 vines per plot, every 2–4wk throughout the trial (40vines per treatment per sample date). Ant feeding activitywas assessed, using the monitoring tubes previouslydescribed, every 1–2wk throughout the trial. At the DryCreek vineyard, we used 7 monitoring tubes in each of the

liquid bait plots and 25 tubes in each control plot. Webalanced the number of vines with tubes dispensing food inthe treatment plots (bait tubes plus monitoring tubes) withthe control plots (monitoring tubes only) because therewere so few mealybugs at this site that we did not want toalter the results by recruiting more ants to feed at the baittreatment. At all sites, mealybug densities were sampled on10 vines per plot, every 4–5wk throughout the season (40vines per treatment per sample date), using the methodsdescribed above. Crop damage was rated at harvest-time,as previously described, except that we increased thenumber of samples to 20 vines per plot, with 7 clustersper vine rated (560 and 420 clusters per treatment at theCarneros and Dry Creek appellation sites, respectively).For all samples, vines were randomly selected from themiddle rows of each plot.

2.6. Statistics

Results are presented as treatment means (7SEM). Forthe visual ant and mealybug counts and ant activity,measured using the monitoring tubes, we compared season-long treatment effects (treatment� sample date) usingrepeated measures analysis of variance (ANOVA) (Systat,2000). Data were transformed (log [x+1]) as needed tostabilize the variance. To discuss the level of treatmentimpact on individual sampling dates in the 2000 and 2001seasons, we used Tukey’s pairwise comparison (Po0:05)(Systat, 2000). For cluster damage, as measured by therating scale, treatment effects were compared in a 2� 2contingency table with treatments separated using Spear-man’s rank order test (Systat, 2000).

3. Results and discussion

3.1. 2000 trials

At the Santa Maria appellation trial, there was asignificant season-long reduction in ant density, comparedwith the control, for the chlorpyrifos barrier spray andboric acid liquid bait treatments, but no effect for theimidacloprid treatment (Fig. 1A). However, treatmentimpact was not great, with mean ant density significantlylower than the control on only 3 of 10 sampling dates forboric acid (17 and 24 July, 14 August) and chlorpyrifos (24July, 23 August and 19 September (Fig. 1A). Moreover, inall treatments the ant density was extremely high through-out the sampling period, with visual ant counts typicallyranging between 150 and 350 ants per 60 s count. In alltreatments, there was a slight reduction in ant density inAugust, which we believe was an artifact of the samplingmethod as we observed the ants spending a greater amountof time tending mealybugs in the grape cluster and less timeforaging on the vine or moving between vines and nests. Atthe second trial site (Edna Valley appellation), the controltreatment was lost (insecticide treatment for mealybugs)and there were no significant differences among the

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Fig. 1. Mean (7SEM) ant densities, in 2000, at the Central Coast trial

sites in the (A) Santa Maria appellation, where season-long ant density

was significantly lower than the control in the boric acid liquid bait

(F ¼ 5:946, df ¼ 1,6, P ¼ 0:051) and chlorpyrifos (F ¼ 5:936, df ¼ 1,6,

P ¼ 0:054) treatments, but not significantly different from the control for

the imidacloprid liquid bait treatment (F ¼ 0:13, df ¼ 1,6, P ¼ 0:731); andin the (B) Edna Valley appellation, where there was no difference among

treatments (F ¼ 1:447, df ¼ 2,9, P ¼ 0:285).

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Fig. 2. In the North Coast (Carneros appellation) 2000 trial, (A) season-

long ant densities (mean7SEM) were not significantly different from the

control for the liquid bait treatments (boric acid: F ¼ 1:979, df ¼ 1,10,

P ¼ 0:190; imidacloprid: F ¼ 1:107, df ¼ 1,10, P ¼ 0:964) or barrier spray(chlorpyrifos: F ¼ 1:199, df ¼ 1,10, P ¼ 0:299), and (B) season-long

mealybug densities (mean7SEM) were not significantly different from

the control for the liquid bait treatments (boric acid: F ¼ 0:564, df ¼ 1,10,

P ¼ 0:470; imidacloprid: F ¼ 1:777, df ¼ 1,10, P ¼ 0:212) or barrier spray(chlorpyrifos: F ¼ 1:981, df ¼ 1,10, P ¼ 0:190). (C) population age

structure is shown for immature ( ¼ second and third instars) and mature

mealybug stages.

K.M. Daane et al. / Crop Protection 25 (2006) 592–603 597

remaining treatments (Fig. 1B). Although the boric acidbait treatment appears to be consistently lower than eitherthe chlorpyrifos or imidacloprid bait treatments in July,there was only a single date (25 July) when ant density wassignificantly lower in the boric acid treatment than theother treatments.

In the North Coast trial (Carneros appellation), therewas no treatment impact on season-long ant densities(Fig. 2A), and on only a single sampling date (13 October)was there a significant reduction in ant density betweenan insecticide treatment (imidacloprid) and the control(Fig. 2A). As was observed at the Central Coast site, antdensity, as measured by visual counts, decreased after July,although we observed an increase in the number of antstending mealybugs in the grape clusters. There wasno treatment effect on season-long mealybug densities(Fig. 2B), and there were two sampling dates (12 Septemberand 13 October) when the mealybug density in aninsecticide treatment (imidacloprid) was significantly lowerthan the control. Mealybug densities in all plots showed adip in August and an increase in September, whichcorresponds to a natural population decline as the summermealybug generation reaches the adult stage (late July) andnew immatures appear (September) (Fig. 2C). Thispopulation age structure, with two generations per year,is typical for the grape mealybug in California vineyards(Geiger and Daane, 2001). Mealybug eggs (in ovisacs),crawlers, and settled first instars produce little honeydew,which may decrease the level of ant tending, therebyincreasing the ants’ interest in other food sources such as a

liquid bait during times of the year when these stagespredominate. There was no treatment effect on damageratings, and there was considerable damage across alltreatments, with more than 70% of all clusters ranked ashaving moderate or high damage (Fig. 3A).That chlorpyrifos had little effect on foraging ant

populations could have resulted from a number of factors.First, Argentine ant densities were extremely high, so theforagers that were initially affected might have beenquickly replaced. Second, on mature vines, the trunkstructure may reduce the effectiveness of barrier insecti-cides: we observed the ant trail often moved in the trunkcrevices, under loose bark, and even inside the trunk inrotted sections that formed hollow corridors. It would be

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severemoderatelownone

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(B)

(C)

Fig. 3. Crop damage ratings were not significantly different among treatments applied in North Coast vineyards in (A) 2000 trials (Pearson’s w2 ¼ 10:586,df ¼ 9, P ¼ 0:305) or (B) 2001 trials (Pearson’s w2 ¼ 16:326, df ¼ 9, P ¼ 0:06), while there was a significant reduction in crop damage in liquid bait

treatments in (C) 2002 trials in the Carneros appellation using 0.3m dispensers (Pearson’s w2 ¼ 44:72, df ¼ 3, Po0:001) and 250ml dispensers, and the

Dry Creek appellation ((Pearson’s w2 ¼ 164:7, df ¼ 3, Po0:001) and the Dry Creek appellation using the 250ml dispenser (Pearson’s w2 ¼ 66:54, df ¼ 3,

Po0:001).

K.M. Daane et al. / Crop Protection 25 (2006) 592–603598

difficult to provide complete coverage on the vine trunk,and especially in these protected areas where the mealybugsare commonly found (Geiger and Daane, 2001). Anybreaks in the barrier may provide potential passagewaysfor ants to slip through (Klotz et al., 2002). Third, once theants gained access to the vine canopy at one incompletelytreated vine, these highly mobile ants could move theircolony to the base of that vine and then forage tonumerous vines within that row by following the vineyardtrellis wires, a behavior we commonly observed.

The poor performance of the barrier spray suggests thatother types of Argentine ant controls are needed.Unfortunately, our initial work with liquid baits showedno impact on the ants and brought into question our baitdelivery and sampling methodologies. The rectangular

dispenser was based on a commercially available dispenserused for urban ant control; the 0.6m PVC dispenser wasdeveloped to make use of materials easily available togrowers and to hold a large quantity of liquid bait, therebyminimizing the labor costs involved in cleaning andrefilling. Ant visitation to both types of dispensersdecreased a few weeks after deployment. This decreasewas especially dramatic at the North Coast site, where thedispensers were not refilled, and where we observed a largenumbers of ants moving up and down the vine trunk whileignoring the dispensers at the base. The results also indicatethat ant foraging behavior changed in late July, whenforagers spent less time removing sugar solution from themonitoring tubes and bait dispensers, and remained forlonger periods in the grape clusters. At the same time, the

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ControlFipronilImidaclopridThiamethoxam

Feb 01 Mar Apr May Jun Jul Aug Sept Oct Nov

Feb 01 Mar Apr May Jun Jul Aug Sept Oct Nov

Feb 01 Apr May Jun Jul Aug Sept Oct NovMar

(A)

(B)

(C)

Fig. 4. Season-long treatment (mean7SEM) impact in the North Coast

2001 trial shows that liquid bait treatments were not significantly different

from the control for (A) ant feeding activity as measured by monitoring

tubes (fipronil: F ¼ 0:080, df ¼ 1,6, P ¼ 0:787; imidacloprid: F ¼ 0:031,df ¼ 1,6, P ¼ 0:865; thiamethoxam: F ¼ 0:268, df ¼ 1,6, P ¼ 0:623), (B)ant density as measured by visual counts (fipronil: F ¼ 0:454, df ¼ 1,6,

P ¼ 0:525; imidacloprid: F ¼ 0:575, df ¼ 1,6, P ¼ 0:477; thiamethoxam:

F ¼ 0:328, df ¼ 1,6, P ¼ 0:588), or (C) mealybug densities (fipronil:

F ¼ 0:072, df ¼ 1,6, P ¼ 0:797; imidacloprid: F ¼ 0:578, df ¼ 1,6,

P ¼ 0:476; thiamethoxam: F ¼ 3:782, df ¼ 1,6, P ¼ 0:100).

K.M. Daane et al. / Crop Protection 25 (2006) 592–603 599

mealybugs were mainly in the second and third instar andadult stage (Fig. 2C), feeding mostly in the grape clusters,and excreting more honeydew than the younger stages.These observations suggest that honeydew from mealybugsfeeding in relatively mature grape clusters or the fluidexudate from the grape, which we observed ants feeding onin the absence of mealybugs, may be more attractive toants than our sugar bait, or even honeydew from mealy-bugs feeding on other parts of the vine.

3.2. 2001 trial

We used the 0.6m bait dispensers to test differentinsecticidal materials and made two changes to oursampling methodology: visual ant counts were conductedyear-round and monitoring tubes were added to follow antfeeding activity during the summer and fall. There was stillno discernable treatment impact, compared with thecontrol, on season-long ant densities as measured byfeeding activity (Fig. 4A) or visual counts (Fig. 4B). Onindividual sampling dates, only thiamethoxam lowered antdensity (October samples) or feeding activity (29 May–8August). Similarly, there was no treatment impact—compared with the control—on season-long mealybugdensities (Fig. 4C) or damage ratings (Fig. 3B).

The results provided guidelines for subsequent trials.First, baits should be deployed earlier in the season. Antswere foraging in February and March (we sampled only onsunny days, with recorded temperatures ranging from 11 to20 1C). Foragers are most active at temperatures between10 and 30 1C (Vega and Rust, 2001) and these temperaturescan be recorded at any time of the year in Californiavineyards. In winter, the grape mealybug population isrelatively sessile—primarily in the egg or first instarstages—and is located under the bark of the trunk orcordon (Geiger and Daane, 2001). Honeydew excretion isalmost non-existent. There follows a sharp increase in antactivity in April (Fig. 4B), corresponding with the increasein mealybug densities as overwintered eggs hatch and theresulting mealybugs begin to move up the vine and beginfeeding–although their honeydew production is still rela-tively slight. Together, these results suggest that an initialdeployment of liquid baits between February and Aprilmight take advantage of the early season ant activity,directing them to the baits when their primary food source(mealybug honeydew) is in short supply. This would placethe liquid baits in the field during a period of increasedbrood production (see Markin, 1970). For example, instudies of ant biology, we found eggs in the ant nest in lateFebruary and early March (Cooper and Daane, unpub-lished data), and the resulting increase in brood, in Marchand April, would lead to an increased demand for foodsupplied to the nest.

A second factor that may have diminished ant visits tothe bait dispensers was the quality of the bait itself. Odorsemitting from the bait stations, especially those with somelevel of sun exposure, indicate the sugar water was

fermenting. The wood shavings provided as substrateinside the dispensers also rotted quickly, producing arancid smell. We therefore decided in 2002, to developsmaller dispensers that could be easily cleaned, refilled, andreplaced.

3.3. 2002 trials

At the Carneros appellation site that used the 0.3m PVCdispensers, there was a significant season-long reduction infeeding activity (Fig. 5A) and ant density (Fig. 5B) in thethiamethoxam bait treatment, compared to the control.There was an even greater reduction in the trial that usedthe 250ml centrifuge-tube dispensers, with significantlylower ant feeding activity (Fig. 6A) and ant density(Fig. 6B). At the Dry Creek appellation site, ant feedingactivity was significantly lower in the liquid bait treatment

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Apr 02 May Jun Jul Aug Sept Oct Nov Dec

Apr 02 May Jun Jul Aug Sept Oct Nov Dec

ControlThiamethoxam

(A)

(B)

(C)

Fig. 5. Season-long treatment (mean7SEM) impact in a North Coast

(Carneros appellation) 2002 trial shows a significant impact of a

thiamethoxam liquid bait delivered in the 0.3m PVC dispensers on (A)

ant feeding activity as measured by monitoring tubes (F ¼ 18:502,df ¼ 1,6, P ¼ 0:005) and ant density as measured by visual counts

(F ¼ 8:648, df ¼ 1,6, P ¼ 0:026). There was no treatment impact on

mealybug density (F ¼ 0:781, df ¼ 1,6, P ¼ 0:411).

20

30

40

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10

0

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10

15

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0.1

50

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ugar

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Apr 02 May Jun Jul Aug Sept Oct Nov Dec

Apr 02 May Jun Jul Aug Sept Oct Nov Dec

10 ControlThiamethoxam

(A)

(B)

(C)

Fig. 6. Season-long treatment (mean7SEM) impact in a North Coast

(Carneros appellation) 2002 trial shows a significant impact of a

thiamethoxam liquid bait delivered in the 250ml tube dispensers on (A)

ant feeding activity as measured by monitoring tubes (F ¼ 28:981,df ¼ 1,6, P ¼ 0:002) and (B) ant density as measured by visual counts

(F ¼ 11:82, df ¼ 1,6, P ¼ 0:014, June–September data). There was no

treatment effect on mealybug density (F ¼ 0:211, df ¼ 1,6, P ¼ 0:662).

K.M. Daane et al. / Crop Protection 25 (2006) 592–603600

(Fig. 7A), although there was no treatment effect on antdensity (Fig. 7B). The reduced activity did not, however,correspond to a reduced mealybug density at either of theCarneros appellation sites (Figs. 5C and 6C) or at the DryCreek appellation site (Fig. 7C). This is probably due atleast in part to the very low mealybug densities found onparts of the vine other than the cluster towards the end ofthe season, in all treatments at all sites. The clusterinfestation ratings provide a better indication of treatmentimpact on mealybug damage and show a significantreduction in crop damage at all three sites (Fig. 3C).

3.4. Development of a commercial program

Our 2002 results indicate that liquid baits can controlArgentine ants in vineyards. Previous research has demon-strated the effectiveness of liquid ant baits in urbanenvironments (Klotz et al., 2003), and has shown that ant

control is a prerequisite to the control of grape and obscuremealybugs in vineyards (Phillips and Sherk, 1991). In the2001 trials, we showed that liquid ant baits reduce cropdamage from ant-tended mealybugs. We did not show atreatment impact on mealybug density. The results haveimportant implications for the commercial development ofant controls in vineyards. The performance of the baitsimproved dramatically after changes in dispenser designand deployment patterns, including deployment of moredispensers, earlier seasonal deployment to take advantageof spring ant foraging, and reduction of bait fermentationby frequently refilling and replacing the dispensers. The250ml dispensers worked best, and we speculate that thismay in part be a result of their position on the vine trunk,where ants were foraging before treatments began, thatresulted in more bait delivered to the colony. The 250mldispensers were also more easily deployed, cleaned,and refilled. However, these smaller dispensers have

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DecJun 02 Jul Aug Sept Oct Nov Jan

Jun 02 Jul Aug Sept Oct Nov Dec Jan

Jun 02 Jul Aug Sept Oct Nov Dec Jan

0

5

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ControlThiamethoxam

(A)

(B)

(C)

Fig. 7. Season-long treatment (mean7SEM) impact in a North Coast

(Dry Creek appellation) 2002 trial shows a significant impact of

thiamethoxam liquid bait delivered in the 250ml tube dispensers on (A)

ant feeding activity as measured by monitoring tubes (F ¼ 8:068, df ¼ 1,6,

P ¼ 0:030). There was no treatment impact on (B) ant density as measured

by visual counts (F ¼ 1:623, df ¼ 1,6, P ¼ 0:250) or (C) mealybug density

(F ¼ 0:324, df ¼ 1,6, P ¼ 0:590).

K.M. Daane et al. / Crop Protection 25 (2006) 592–603 601

disadvantages, mainly that with their small volume theyrequire more frequent maintenance to refill and re-deploy.Furthermore, the number of dispensers used per ha wasunrealistically high with respect to large-scale commercialapplications. In contrast to urban systems, where baitdispensers might be placed strategically around buildings(Klotz et al., 1998), the high ant densities encountered invineyards would reduce the effectiveness of a small numberof low-volume liquid bait dispensers. As farm managementrequires low-maintenance and low-cost ant controls, thecurrent design is impractical for large scale use throughoutthe season. One possible option is to apply baits intensivelyfor short periods during critical periods in the developingnest population. For example, deployment of a highnumber of small bait dispensers in April and May, whichwould target the developing reproductive brood, may havethe same, or an even stronger, impact on ant density andmealybug damage as deployment of fewer dispensersthroughout the season.

The development of a commercial program requires thattwo questions be addressed. First, how many bait stationsper ha are needed? This can be determined with informa-tion about how far the ants forage for food, how large thepopulation is at any one site, and how much liquid bait adispenser can effectively hold. Most of the ant colony willforage within small, known territory (Nonacs and Soriano,1998), increasing searching range upon the discovery ofbetter food sources by the small portion of the populationthat forages outside the normal range. In our studies,dispensers were deployed at rates of approximately 85, 160,175, 185 and 620 per ha, placing dispensers never more than30m apart and sometimes as close as 6m from aneighboring dispenser. For example, at the Dry Creekappellation site (2002 trial) we deployed 620 dispensers perha. Many of the vines at this site dated to 1900; because ofthe value of the vines and the grower’s interest insustainable farming practices already in use, the grower–-collaborator wanted to investigate alternatives to organo-phosphate sprays to reduce the Argentine ant andmealybug problems at their inception and this trial wasdesigned without regards to commercial costs. In fact,deployment rates ranging from 5 to 20 dispensers per haare probably more economically feasible. We also foundthat at many of our trial sites the Argentine ant populationdensities were remarkably high, which would certainly limitthe effectiveness of a low-dose, liquid bait program if manyliters per ha of liquid bait would feed only a small portionof the population. For example, Reierson et al. (1998)showed that each gram of sugar water removed frommonitoring tubes represents 3300 ant visits, and at ourCarneros appellation sites we commonly found 430 g per dof sugar water removed from monitoring tubes during theJune–July period, the equivalent of 30,000 ant visits per24 h. Not surprising at these densities, the 250ml baitdispensers were often emptied in 1wk. We are currentlytesting 3-l dispensers and lower deployment rates.Second, when should the dispensers be deployed? The

seasonal deployment pattern will affect both the successand cost of any liquid bait program. Year-round deploy-ment would be most effective, but may be impossible inpractice. In all trials, we noted a decrease in recorded antdensity or feeding activity from July to September, whichwas sometimes quite dramatic (Fig. 4A). However, duringthe same period we observed an increase in ant activity inthe grape clusters. The drop in dispenser visitation willreduce the effectiveness of the bait dispersion during thisperiod, and brings in to question the relative attractivenessof 25% sugar water compared with honeydew excreted bymealybugs feeding inside a grape cluster. Also, during thespring period, we observed less honeydew production frommealybugs feeding primarily on the trunk and canes, ascompared with honeydew production during the summermonths when mealybugs were feeding in the grape clusters.Therefore, bait dispensers may provide a food source whenants are active and mealybugs are producing littlehoneydew. The effectiveness of a control program might

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also be increased by targeting areas of the vineyard withhigh ant activity for more intensive bait deployment. At theCarneros site, for instance, we observed higher ant activityin parts of the vineyard with moister soils, and it may bemost economical to concentrate bait treatments in suchareas.

We initially sought to compare various materials and,over the 3-year period, used fipronil, thiamethoxam,imidacloprid, and boric acid in a liquid bait, as well as abarrier application of chlorpyrifos. The less effective trapdesign and deployment used in the comparison trials in2000 and 2001 renders questionable any conclusionsregarding the relative effectiveness of the active ingredientsdelivered in the liquid baits. We can, however, concludethat the chlorpyrifos barrier treatment, which is thestandard treatment for Argentine ants in vineyards, hadlittle or no long term impact on ant densities. Baits aremore likely to provide effective Argentine ant control invineyards and have the additional advantage of meetingthe needs of sustainable farming practices.

Acknowledgments

We thank the Domaine Chandon, Edna Valley, Para-gon, Ridge, Rutherford, and White Hills personnel for useof their vineyards and help with management of the fieldsites; Julie Fallon, Kevin Fingerman, Sasha Mortezaei, andNicole Vajda for help with field and laboratory work; andErik Nelson for reviews of earlier drafts of this manuscript.Funding was provided by the American Vineyard Founda-tion, Viticulture Consortium, the California CompetitiveGrants Program for Enology and Viticulture, and theCentral Coast Vineyard Team.

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