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Native braconid parasitism of theTarnished plant bug (Hemiptera:Miridae) in Southern OntarioA. Bruce Broadbent a , Simon Lachance b , Mark K. Sears c & HenriGoulet da Southern Crop Protection and Food Research Centre, Agriculture& Agri-Food Canada, London, Ontario, Canadab Collège d'Alfred de l'Université de Guelph, Alfred, ON, Canadac Department of Environmental Biology, University of Guelph,Guelph, ON, Canadad Insect Biosystematics, Agriculture & Agri-Food Canada, Ottawa,ON, CanadaPublished online: 18 Jan 2007.
To cite this article: A. Bruce Broadbent , Simon Lachance , Mark K. Sears & Henri Goulet (2006):Native braconid parasitism of the Tarnished plant bug (Hemiptera: Miridae) in Southern Ontario,Biocontrol Science and Technology, 16:7, 687-698
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Native braconid parasitism of the Tarnished plant bug(Hemiptera: Miridae) in Southern Ontario
A. BRUCE BROADBENT1, SIMON LACHANCE2, MARK K. SEARS3,
& \HENRI GOULET4
1Agriculture & Agri-Food Canada, Southern Crop Protection and Food Research Centre,
London, Ontario, Canada, 2College d’Alfred de l’Universite de Guelph, Alfred, ON, Canada,3Department of Environmental Biology, University of Guelph, Guelph, ON, Canada, and4Agriculture & Agri-Food Canada, Insect Biosystematics, Ottawa, ON, Canada
(Received 2 March 2005; returned 18 May 2005; accepted 8 December 2005)
AbstractThree agricultural regions in southern Ontario (London, Niagara and Guelph) were sampledweekly from May to September for Lygus spp. and their parasitoids in 1998, 1999 and only oneregion in 2000. Alfalfa was the primary crop sampled, including both ‘‘clean’’ and weedy alfalfafields. Lygus lineolaris (Palisot de Beauvois) accounted for 99% of all Lygus spp. collected bysweepnet. Lygus lineolaris had two generations (mid-June and late July) on alfalfa with a partialthird generation in early September which contributes to the over-wintering adults. In 1998 and2000, an early first generation peak of Lygus nymphs was observed in mid-May on the weeds,chickweed and shepherd’s purse. Overall rates of parasitism (from dissections) for nativeparasitoids were consistent from year to year and the means of the three regions in each growingseason were below 11%. Both nymphs and adult Lygus were parasitized, with the highly mobileadults being a potential means of dispersing the parasitoids. In general weedy fields were morehighly parasitized than fields of weed-free alfalfa. Six species of native braconid parasitoids werecollected from L. lineolaris in southern Ontario (in decreasing order of occurrence): Peristenuspallipes (Curtis), Peristenus pseudopallipes (Loan), Leiophron lygivorus (Loan), L. solidaginisLoan, L. uniformis (Gahan), and Leiophron sp. near brevipetiolatus Loan. The large populationsof Lygus and the low percent baseline parasitism in southern Ontario, particularly of secondgeneration Lygus , support the need for introduction of a multivoltine parasitoid species in thisregion.
Keywords: Lygus, tarnished plant bug, Peristenus, braconid, parasitism
Introduction
The Tarnished plant bug (TPB), Lygus lineolaris (Palisot de Beauvois) (Hemiptera:
Miridae) is an important native agricultural pest in North America. It is highly
polyphagous and damages vegetables, fruits, greenhouse crops, canola and legume
crops, primarily those grown for seed such as alfalfa (Medicago sativa L.). In Ontario,
Correspondence: A. Bruce Broadbent, Agriculture & Agri-Food Canada, Southern Crop Protection and
Food Research Centre, 1391 Sandford St., London, Ontario, Canada N5V 4T3. Tel: 1 519 457 1470, ext.
251. E-mail: [email protected]
ISSN 0958-3157 print/ISSN 1360-0478 online # 2006 Taylor & Francis
DOI: 10.1080/09583150600699853
Biocontrol Science and Technology, 2006; 16(7): 687�698
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we estimate Lygus damage to be about 5% of both fruit and vegetable crops resulting
in �/Can.$12 million in annual losses (Broadbent et al. 2002). In Saskatchewan the
$50 million alfalfa seed industry can be completely destroyed by Lygus and other plant
bugs if not treated (Soroka 1997) and in southern Alberta, in 1997, although
200 000 ha of canola were sprayed (Mason & Soroka 1998) an estimated 20% of the
crop, valued at �/$70 million, was lost due to Lygus damage.
Surveys of native parasitoids of the TPB have been carried out in parts of North
America (Loan 1965, 1970, 1974, 1979, 1980; Clancy & Pierce 1966; Graham et al.
1986; Day 1987; Braun et al. 2001). In southern Ontario, more than 15 species of
nymphal parasitoids (Hymenoptera: Braconidae) were collected from various mirid
species on a variety of host plants in the Belleville area (Loan 1970, 1980). The rate of
parasitism varied between 16 and 64%, depending on the species involved and time of
the season. Six species of nymphal parasitoids attacking L. lineolaris have been
collected from eastern Canada: Peristenus pallipes (Curtis), P. pseudopallipes (Loan), P.
digoneutis Loan, P. conradi Marsh, Leiophron lygivorus Loan, and L. uniformis (Gahan)
(Loan 1970, 1980; Broadbent et al. 1999). Two of these species, P. digoneutis and P.
conradi , are introduced European parasitoids and were first collected in Quebec
(Broadbent et al. 1999), having moved north from their release point in New Jersey,
USA. Day (1996) reported a decrease by 75% of TPB populations in alfalfa following
release and establishment of the bivoltine P. digoneutis in the northeastern United
States.
Releases in Ontario of European parasitoids to control the TPB may therefore be an
option to enhance control of this highly mobile and polyphagous pest. Programs are
still in progress to introduce European parasitoids in various locations in North
America (Kuhlmann et al. 1998). However, baseline composition of native parasitoid
populations will help in evaluating the impact and establishment of any exotic species.
The objective of this study was to identify Lygus species, especially Lygus lineolaris ,
and their native parasitoids on alfalfa and weedy sites. The data collected from several
sites over 2�3 years will serve as a baseline for subsequent release and establishment of
one or more exotic parasitoids for control of TPB in southern Ontario.
Materials and methods
Three agricultural regions in southern Ontario (near the cities of London, Niagara,
and Guelph) were sampled for mirids and their parasitoids in 1998 (six, 15 and nine
fields, respectively), 1999 (six, 14 and seven fields, respectively), and 2000 (only
London alfalfa). Sampling was done weekly from May to September at the various
field sites. Alfalfa was the primary host plant selected as it is an important crop for
Lygus population build-up and it occupies a large area in Ontario of approximately 2.5
million acres (OMAF 2002). Weedy alfalfa sites and uncultivated weedy sites were
also selected with a variety of common weeds, such as redroot pigweed (Amaranthus
retroflexus L.), shepherd’s purse (Capsella bursa-pastoris (L.) Medic.), chickweed
(Stellaria media (L.) Vill.), wild mustard (Sinapis arvensis L.), lamb’s quarters
(Chenopodium album L.), chamomile (Matricaria spp.), goldenrod (Solidago spp.),
Canada fleabane (Erigeron canadensis L.), wild carrot (Daucus carota L.), tufted vetch
(Vicia cracca L.), viper’s bugloss (Echium vulgare L.), as well as grasses such as timothy
(Phleum pratense L.). A field was considered ‘‘weedy’’ when a visual inspection
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suggested �/10% of the field was comprised of weeds and ‘‘clean’’ if B/10%. Fields
weedy or not, were not treated with pesticides during the season.
Sampling involved two or three sets of 50 1808 sweeps per field with a 40-cm
diameter sweep net. The back and forth motion of the sweep net was considered to be
two sweeps. Mirid species, stage and number were recorded for each sample.
Dissections of late instar nymphs (N3�N5) and adult plant bugs (if �/10 each per
field) were done under a binocular microscope in order to establish percent parasitism
(defined as the percent of a mirid species containing hymenopteran larvae on
dissection). Up to a maximum of 30 adults and 30 nymphs were dissected per species
per 50 sweeps. For each year sampled, we determined mean parasitism rates per week
per field. To show trends in Lygus populations, data from fields were pooled and
presented as one graph for each region. For the Niagara region, three weedy alfalfa
fields that were sampled consistently each week were pooled to graph the season-long
Lygus number and parasitism rates.
Rearing a subsample of mirids collected in fields with high rates of parasitism was
also undertaken to obtain adult wasps for species identification. Parasitized nymphs
were reared (16:8 h L:D, 248C) on lettuce and beans in a screened-bottom, plexiglass
tub (15�/9.5 cm, height�/diameter) with moist vermiculite as a pupation medium
placed underneath in a 500-mL clear plastic container (Whistlecraft et al. 2000). The
cocoons obtained were kept at 248C for a minimum of 3 weeks to allow for emergence
of bivoltine or multivoltine species. Remaining cocoons were subsequently transferred
to 148C for 7 days, then 78C for 7 days, and finally to 28C for a minimum of 5 months
in order to complete diapause. Care was taken to keep the vermiculite moist during
the diapause period. Cocoons were transferred back to 248C for wasp emergence.
Specimens were identified by one of the authors (HG).
Mean parasitism for an entire field consisted of pooled data over the several dates
sampled. The impact of host plant type on parasitism rate was assessed for the Guelph
area by comparing percent parasitism from clean alfalfa fields to weedy fields. Only
fields with more than 100 nymphs dissected were considered for the analysis, for a
total of seven fields for each host-plant over the 2 years. Before the statistical analysis,
the percent parasitism was log (x�/1) transformed, but back transformed means are
presented. Average parasitism rates from clean alfalfa and weedy sites were subjected
to an ANOVA using the general linear models procedure. Mean numbers of parasitism
were separated using Tukey’s studentized range test. Differences among treatments
were considered significant when P 5/0.05 (SAS Institute Inc. 2001).
Results
Mirid populations
Lygus lineolaris was the predominant mirid collected at all three regions in SW
Ontario, accounting for 99% of the Lygus collected. A few specimens of L. plagiatus
Uhler were collected at the Talbotville site, near London, on weeds (4 and 26 August
1998) and at the Southern Crop Protection and Food Research Centre (SCPFRC)
farm, London, on weedy alfalfa (13 September 2000). The mean number of nymphs
and adults of TPB collected per 50 sweeps varied greatly depending on the site, the
date of sampling, and the host plants sampled. Mean TPB populations per week are
presented for London fields pooled (Figure 1), Guelph fields pooled (Figure 2) and
Niagara fields pooled (Figure 3). Two generations of TPB nymphs were generally
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observed on alfalfa. Fields of alfalfa containing a large proportion of weeds usually had
higher parasitism levels for nymphs (Table I, for Guelph). In 1998 and 2000, in weeds
bordering an alfalfa field in London, the first generation peak of nymphs appeared in
mid-May on chickweed and shepherd’s purse, the progeny of over-wintering adult
TPB which laid eggs before the alfalfa had greened up. A third generation of TPB
nymphs was occasionally seen on alfalfa in early September (Figures 1 and 4), which
presumably gave rise to the over-wintering adult population.
Adult TPB numbers fluctuated greatly over the spring and summer months, making
it difficult to determine a peak period of adult density. There were usually two or three
overlapping generations of adults on alfalfa. In some fields, a distinct large
overwintering population of adult TPB was detected in early September (Figure 4).
In London’s weedy alfalfa that was strip cut during the season, the Lygus population
was considerably higher than in commercial alfalfa fields by a factor of approximately
6-fold.
Parasitism
The overall baseline parasitism of Lygus in the summers of 1998 and 1999 was low,
with B/11% parasitism for the mean of the three sites in each season (Table II).
Maximum parasitism rates recorded in specific fields ranged between 20 and 60% for
both nymphs and adults (Table II). Although parasitized as nymphs, some parasitized
Lygus do develop to the adult stage.
London region. Parasitism for all sites sampled, in 1998 and 1999, respectively, were
7.6 and 5.5% for Lygus nymphs dissected and 1.5 and 4.0% for Lygus adults
dissected. When totalled for both nymphs and adults, very consistent means of 5.0
and 4.6% parasitism for 1998 and 1999 were observed (Table II). Parasitized nymphs
were found from 1 June to 12 August in 1998, from 4 June to 15 September in 1999,
and from 5 June to 7 August in 2000 (Figure 1). Parasitized adults were found from 30
June to 11 August in 1998, from 8 June to 15 September in 1999 and from 19 June to
7 August in 2000 (Figure 1).
In 1998, maximum parasitism for nymphs was observed in a weedy alfalfa field on
4 June (50%) and for adults was in a weedy alfalfa field on 31 July (20%) (Table II).
In 1999, maximum parasitism for nymphs was observed in a weedy alfalfa field on 8
June (30%) and for adults was in an unsprayed strawberry field on 15 June (47%)
(Table II).
Table I. Overall parasitism of Lygus lineolaris nymphs and adults in alfalfa fields versus weedy fields in the
Guelph area (1998 and 1999).
Lygus lineolaris nymphs Lygus lineolaris adults
Host-plant No. fields No. dissected No. paras. % paras.* No. dissected No. paras. % paras.**
Alfalfa fields 7 880 19 2.2a 1844 45 2.4a
Weedy fields 7 1278 95 7.4b 1827 134 7.3a
Averages (% parasitism) within the same column followed by the same letter are not significantly different
(P �/0.05; Tukey’s multiple comparison test). Fields with more than 100 individuals dissected were
included. *P�/0.0177, F�/7.72, df�/12. **P�/0.1282, F�/2.67, df�/12.
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Guelph region. Parasitism for all sites sampled, in 1998 and 1999, respectively, were
6.3 and 3.9% for Lygus nymphs and 3.7 and 5.9% for Lygus adults (Table II). When
totalled for both nymphs and adults, very consistent means of 4.7 and 5.2% parasitism
for 1998 and 1999 were observed (Table II). Parasitized nymphs were found from 18
May to 17 August in 1998 and from 1 June to 17 August in 1999 (Figure 2).
0
5
10
15
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25
30
% P
aras
itis
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160
181998
1999
2000
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No. nymphs
No. adults
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ps
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13 20 27 4 11 18 25 1 8 15 22 29 6 13 20 27 3 10 17 24 31 7 14
Date
% P
aras
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0
10
20
30
40
50
Mea
n n
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fLyg
us/5
0 s
wee
ps
May June July August Sept.April
Figure 1. Lygus number and parasitism rates, weedy and clean alfalfa fields pooled, London area 1998 (n�/
6) top ; 1999 (n�/6) middle ; and 2000 (n�/5) bottom . Note: In all the season long graphs (mid-April to mid-
September 1998, 1999, and 2000), note that the scale of the Y-axis (Lygus per 50 sweeps) changes
considerably.
Native braconid parasitism of the Tarnished plant bug 691
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Parasitized adults were found from 1 June to 24 August in 1998 and from 15 June to
10 August in 1999 (Figure 2).
In 1998, maximum parasitism for nymphs was observed in a weedy field on 27 July
(39%) and for adults was in an alfalfa field on 24 June (39%) (Table II). In 1999,
maximum parasitism for nymphs was observed in an alfalfa field on 16 June (19%)
and for adults was in an alfalfa field on 23 June (39%).
Niagara region. Parasitism for all sites sampled in 1998 and 1999, respectively, were
9.3 and 9.9% for Lygus nymphs and 12.4 and 7.5% for Lygus adults (Table II). When
totalled for both nymphs and adults, relatively consistent means of 10.7 and 8.4%
parasitism for 1998 and 1999 were observed (Table II). Parasitized nymphs were
found from 3 June to 20 August in 1998 and from 31 May to 2 September in 1999
(Figure 3). Parasitized adults were found from 19 June to 2 September in 1998 and
0
5
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30
35
40
% P
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351998
1999
Mea
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ep
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% par. adults
No. nymphs
No. adults
0
10
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30
4 11 18 25 1 8 15 22 29 6 13 20 27 3 10 17 24 31 7 14
Date
% P
aras
itis
m
0
10
20
Mea
n n
umbe
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fLyg
us/5
0 sw
ee
ps
May June July Augus Sept.t
Figure 2. Lygus number and parasitism rates, weedy and clean alfalfa fields pooled, Guelph area 1998 (n�/
9) top and 1999 (n�/7) bottom .
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from 3 June to 2 September in 1999 (Figure 3). A supernumerary parasitoid larva
within the host along with the primary parasitoid was detected on only three occasions
in 1999 from the 2222 nymphs dissected, suggesting a low incidence of super-
parasitism in nature.
In 1998, maximum parasitism for nymphs was observed in a weedy (especially
ragweed, Ambrosia artemisiifolia L.) alfalfa field on 11 August (43%) and for adults
was in a weedy alfalfa field on 27 July (34%) (Table II). In 1999, maximum parasitism
for nymphs was observed in a weedy alfalfa field on 9 August (53%) and for adults was
in a weedy alfalfa field on 18 June (60%) (Table II).
The least disturbed habitats had the highest incidence of parasitism as shown by
data from Guelph, where the host density decreased following the cutting of a weedy
alfalfa field on 13 July 1998 and the late July, early August population of parasitoids
was absent (Figure 4). The same field in 1999 remained uncut and a large number
0
5
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30
35
40
% P
aras
itis
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0
20
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1998
1999
Mea
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ps
% par. nymphs
% par. adults
No. nymphs
No. adults
0
10
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40
27 4 11 18 25 1 8 15 22 29 6 13 20 27 3 10 17 24 31 7 14
Date
% P
aras
itis
m
0
5
10
15
20
25
30
35
40
45
Mea
n n
umbe
ro
fLyg
us/5
0 s
wee
ps
May June July August Sept.
Figure 3. Lygus number and parasitism rates, weedy and clean alfalfa fields pooled, Niagara area 1998 (n�/
3) top and 1999 (n�/3) bottom .
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of parasitized Lygus nymphs and adults were collected in late July, early August
(Figure 4).
Parasitoid species
Collections of nymphs and adult plant bugs in fields with higher parasitism levels were
made at irregular intervals. Rearing of these samples produced adult wasps for species
identification. Approximately 10,000 nymphs and adults of L. lineolaris were collected
for rearing in 1997, 1998 and 1999. Despite technical difficulties in long-term rearing,
six species of native parasitoids were identified from the survey: Peristenus pallipes , P.
pseudopallipes , Leiophron lygivorus , L. solidaginis Loan, L. uniformis and Leiophron sp.
near brevipetiolatus Loan.
Voucher specimens of all species were deposited in the Canadian National
Collection of Insects, AAFC, Ottawa. Peristenus pallipes was the most common
0
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30
35
40
% P
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501998
1999
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ep
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% par. adults
No. nymphs
No. adults
Harvested
0
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35
4 11 18 25 1 8 15 22 29 6 13 20 27 3 10 17 24 31 7 14
Date
% P
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0
10
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30
40
50
Mea
n n
umbe
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fLyg
us/5
0 s
we
eps
May June July August Sept.
Figure 4. Lygus number and parasitism rates in a weedy alfalfa field, Guelph area 1998 (Field cut in mid-
summer) top and 1999 (Field not cut for entire collecting period) bottom .
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parasitoid collected and reared early in the summer (from collections made from 5
June to 17 July in all three summers). Peristenus pseudopallipes was usually collected
starting 2 weeks after the last collection of P. pallipes . During the three summers, we
collected P. pseudopallipes from 16 July to 14 August. Leiophron lygivorus emerged from
nymphs collected in July, August and September, mostly from various weedy fields.
The other species were found to be only occasional, and were not collected in large
enough numbers to establish any relationship to their period of occurrence. Few
parasitized nymphs and adult TPB were found in alfalfa in late summer.
A few specimens of the hyperparasitoid Mesochorus spp. (Hymenoptera: Ichneu-
monidae) emerged from the reared samples of 1999 the Niagara collection [12
hyperparasites (15.8%) emerged from 76 primary parasitoids]. A few parasitic
nematodes were also found during dissection. These nematodes were almost
exclusively dissected from the July/August collected adult TPB in the Guelph region.
In 1999, eight tachinid (Diptera: Tachinidae) larvae and one adult were dissected
from TPB adults collected in the Niagara and London regions.
Discussion
Lygus lineolaris was the predominant mirid collected at all three sampling sites in
southern Ontario. There were two or three overlapping generations of Lygus on alfalfa
in these regions. The few specimens of Lygus plagiatus found in this study, were
usually recovered from ragweed in the fall in Ontario (Schwartz & Foottit 1998).
However, L. lineolaris is the primary mirid of concern in Ontario agriculture causing
economic crop damage.
In this study, the six species of native larval braconid parasitoids collected from
Lygus lineolaris in southern Ontario were (in decreasing order of occurrence):
Peristenus pallipes (Curtis), P. pseudopallipes (Loan), Leiophron lygivorus (Loan), L.
solidaginis Loan, L. uniformis (Gahan), and Leiophron sp. near brevipetiolatus Loan.
Further taxonomic studies by H. Goulet indicate that the spring P. pallipes complex in
fact contains several different species. A detailed study of host plants may indicate
Table II. Parasitism of Lygus lineolaris nymphs and adults collected from three regions in Ontario 1998 and
1999.
Region
and year Date
No.
sites
No.
nymphs
dissected
%
parasitism Range
No.
adults
dissected
%
parasitism Range
Total %
Parasitism
nymphs�/
adults
1998
London 19 May�2 Sept 6 764 7.6 0�50 586 1.5 0�20 5.0
Guelph 27 May�9 Sept 9 1360 6.3 0�39 2104 3.7 0�39 4.7
Niagara 6 May�2 Sept 15 2652 9.3 0�43 2151 12.4 0�34 10.7
Mean 7.7 6.0 6.8
1999
London 3 May�31 Aug 6 2123 5.5 0�30 2890 4.0 0�47 4.6
Guelph 18 May�31 Aug 7 932 3.9 0�19 1713 5.9 0�39 5.2
Niagara 5 May�17 Aug 14 2222 9.9 0�53 3653 7.5 0�60 8.4
Mean 6.4 5.8 6.1
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more specific associations between the parasitoids, their hosts and their preferred
habitats.
The univoltine species P. pallipes complex was common during the peak of first
generation TPB nymphs. Peristenus pseudopallipes was observed parasitizing the second
generation TPB nymphs at the end of July and the beginning of August. The
multivoltine species, L. lygivorus , was predominantly found in the Guelph region, and
was more noticeable at the end of the second generation Lygus nymphs, in late
August/early September, similar to the Leiophron sp. near brevipetiolatus , which is most
likely also a multivoltine species. This may explain the higher level of parasitism of
second generation Lygus in the Guelph region compared to the London area.
Overall parasitism levels were consistent for the 3 years studied (Table II), although
greatly influenced by different host-plants and time of sampling. There was far greater
parasitism in June in the alfalfa and weedy fields surveyed than in July or August in the
Guelph region. Fields with a mixture of weeds were significantly more highly
parasitized than alfalfa fields (Table I), probably due to the continuous source of
nectar from flowering weeds available for the parasitoid adults. Fields with continuous
growth of plants (weedy fields mostly or partly cut alfalfa) usually sustained parasitoid
populations for most of the spring/summer, whereas cutting of alfalfa seemed to
drastically reduce both host and parasitoid populations in the Guelph field studied
over 2 years (Figure 4). Less parasitism by P. pseudopallipes (end of July/early August)
was observed in alfalfa fields versus weedy fields, which is in accordance with data
from Shahjahan and Streams (1973) and Lim and Stewart (1976). Host-plant
preference might explain, in part, this difference, because P. pseudopallipes seems to be
attracted to Erigeron spp. which would serve as a nectar source (Shahjahan 1974). As
well, L. lineolaris feeds on a wide variety of weeds (Snodgrass et al. 1984) and is often
found in very high densities in weeds surrounding agricultural fields (Cleveland
1982). Higher levels of parasitism observed in the Niagara region may be related to the
greater number of fields sampled that were higher in weed content compared to the
fields sampled in the Guelph and London area.
It was important to dissect adults collected from the fields, as a significant
proportion of them were parasitized in both first and second generation TPB. Percent
parasitism of adults has not often been reported in other studies. Parasitized Lygus
adults may be an important means of dispersing parasitoids due to the high mobility of
Lygus adults compared to Lygus nymphs or adult parasitoids. In Europe, parasitized
Lygus adults were not found in a 3-year study (White 2000). But more recently
Ramert et al. (2005) has shown parasitism of adult Lygus in Sweden.
Most larvae of P. pseudopallipes will emerge from the late instars of TPB nymphs
rather than from the adult (Lim & Stewart 1976). However, some parasitized adults
were found in July/August, which suggests another parasitoid acting in conjunction
with P. pseudopallipes , probably Leiophron lygivorus . However, very few L. lygivorus
were reared from collections. Further refinement of rearing techniques or the
availability of molecular marking technologies (Erlandson et al. 2003) would greatly
enhance our capabilities of identifying parasitoids, especially in the larval stage, and
understanding their host-relationships.
Peristenus digoneutis and P. stygicus , both European parasitoids, were better in-host
competitors than native species found in Ontario when tested in the laboratory
(Lachance et al. 2001). The high number of TPB and generally low percent parasitism
in many of the fields sampled in this study suggests a need for further studies on the
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potential introduction of exotic natural enemies for TPB into Ontario. The baseline
data gathered in this study will aid in evaluating the impact of one or more released
exotic species of parasitoids on TPB and the native parasitoid fauna. The introduction
and establishment of the bivoltine parasitoid Peristenus digoneutis Loan in north-
eastern USA (Day et al. 1990, 1998; Day 1996), and the subsequent increase in
percent parasitism of TPB suggests that further introductions in areas where P.
digoneutis are not present could enhance control of TPB. Peristenus digoneutis and P.
conradi were collected for the first time in Canada during field collections of TPB in
southern Quebec in June 1998 and 1999 (Broadbent et al. 1999). Levels of Lygus
parasitism in alfalfa in this region of southern Quebec were very high in 2000�2002
due to P. digoneutis (P. Mason, pers. comm.). The westward spread of this parasitoid is
being monitored in Ontario.
Acknowledgements
We thank Lola Gualtieri, Lou Verdon, and Don Marshall for excellent technical
assistance. This work was funded in part by an Ontario Research Enhancement
Program [OREP] grant.
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