Insect Pests of Winter Canola with Emphasis on Cabbage Seedpod Weevil and Pod Damage Assessment R. Ward, E. Cebert , K. Ward and S. KumarDepartment of Plant and Soil Science, Alabama A&M University Normal, AL 35762ABSTRACT
The cabbage seedpod weevil, Ceutorhynchus obstrictus (Coleoptera: Curculionidae), is the most dominant and destructive insect pest on winter canola (Brassica napus) in north Alabama. Secondary pests that also warrant monitoring include diamondback moth (Plutella xylostella), tarnished plant bugs (Lygus lineolaris), false chinch bug (Nysius sp.), thrips (Frankliniella sp.), aphid complex, flea beetles (Phyllotreta spp.) and root maggot (Delia sp.). Aphids have been reported to severely damage canola in other areas in the southeast; however, they have not presented any major problem in our evaluation plots. Cultivars evaluated in this study were Jetton, Celsius, Explorer and Banjo which were selected for their superior agronomic qualities. Each cultivar was planted following standard planting practices in 100 x 50 m2 plots with four replications arranged in randomized complete block design. Insects were sampled weekly by making 30 sweeps across each canola plot at flowering period. Changes in population levels of cabbage seedpod weevil (CSPW), diamondback moth (DBM), tarnished plant bugs (TPB) and clover stem borer (CSB) were determined. Results showed significant variation in CSPW, TPB and DBM counts over time. DBM population followed a steady decline throughout the reproductive stage of the plant. CSB infestation was insignificant on all cultivars. Assessment of damage caused by cabbage seedpod weevil was done on pods removed from the primary raceme from 36 plants (4 cultivars). Pods were examined for exit holes and other signs of larval feeding or presence of larvae. Corresponding yield losses from pod damage were reported.
As in previous years, cabbage seedpod weevil remained the most dominant and most destructive insect pest on canola in north Alabama during 2004-05 cropping season.
Infestation by CSPW did not differ significantly among cultivars (Banjo, Explorer, Celsius and Jetton) but significantly over time.
CSPW populations peaked in the second to third week in April and first week in June; similar observations were recorded in 2002 (Fig. 6).
Considerably high infestation of diamondback moth and tarnished plant bug was observed in 2005; notably, DBM and TPB were not listed among those collected in 2002 (Fig. 5).
Banjo sustained lowest infestation by DBM (Fig. 8). Highest mean DBM was observed on Celsius. DBM peaked during the first two weeks in May but followed a continuous decline thereafter; it was not detected first week in June.
Relatively high numbers of TPB were detected in mid-May when plants were at 100% pod stage. TPB population continued to increase until pods were ready for harvest. TPB is a serious pest of cotton; canola, therefore, could serve as a reservoir for TPB in the spring time prior to its migration into adjacent cotton fields.
Damage by cabbage seedpod weevil on canola could be more potentially severe due to direct feeding on seeds. However, feeding damage also predisposes pods to early shattering, resulting in higher seed losses. Also feeding damage facilitates disease infection; exit holes are entry points for disease organisms to penetrate plant tissues. Additionally, water seepage into the pods through exit holes induce premature seed germination (vivipary).
Feeding damage by CSPW was measured by the presence of exit holes and other feeding signs on canola pods. Mean number of pods that had 1and 2 exit holes varied significantly among cultivars (Cebert et al. 2004); no significant difference was detected for those with 3 exit holes. The number of exit holes is useful in measuring degree of host plant resistance to CSPW in canola (Dosdall and Kott 2006).
Forty five percent of sample pods had exit holes produced by CSPW; 38.6 %, 5.6% and 1.1% of pods had only one, 2 and 3 exit holes, respectively.
Based on the average seed losses calculated from pods sustaining 1-, 2- and 3-exit holes (40, 91 and 99 % seed weight reduction, respectively) (Cebert et al. 2004), estimated average losses were 16.0, 5.1 and 1.0 %, respectively, or an average of 22.1% total yield reduction.
INTRODUCTIONWinter canola has been commercially grown in northern Alabama for a brief period nearly two decades ago when the seed crushing plant was in operation in Chatanooga, TN. For lack of market after the crusher closed down, farmers abandoned the crop. With growing concern for dissipating fossil fuels, interest in canola, not only as a source of edible oil but also as a renewable product for biodiesel production, has reached a new peak. Canola is a valuable source of edible oil that is often compared to olive oil in healthful quality, but it can also compete well with soybean oil as feedstock for biodiesel. Canola is not grown in large acreages anywhere in the southeastern U.S. at this time but with the growing demand for renewable resources, demand for canola is on the rise. A number of promising canola cultivars have been identified as suitable for growing in north Alabama. Our evaluation included four cultivars selected for superior agronomic characteristics. There are several insect pests associated with winter canola in north Alabama, e.g., cabbage seedpod weevil (CSPW) (Figs.1), false chinch bug, diamondback moth (DBM) (Fig.11), flea beetles, tarnished plant bug (TPB) (Fig. 12), cabbage maggots, thrips, aphids, etc.). These The cabbage seed pod weevil (CSWP), Ceutorhynchus obstrictus Marsham (Coleoptera: Curculionidae), is the most dominant and the most destructive pest on canola in the area. Adult weevils lay eggs on flower buds, young pods, flowers and shoots of canola. Upon hatching, the larvae (Fig. 4) find their way into the pods, feed on the young seeds and leave the pods through exit holes when grubs are ready to pupate. Exit holes are easily recognizable on damaged pods (Fig. 2). The severity of feeding damage on seeds and exit holes on canola pods by CSPW could be exacerbated by moisture seepage through the exit holes causing the seeds to prematurely germinate, a phenomenon called vivipary (Fig. 3); this results in increased seed losses. Infestation levels by other pest species were generally insignificant except for their occasional resurgence. Based on their relative abundance last season, secondary pests that may warrant monitoring include diamondback moth and tarnished plant bugs. This presentation reports on CSPW, DBM and TPB infestation levels during 2004-05 cropping season. MATERIALS AND METHODS
General ProcedureExperimental plots were established at Alabama A&M Universitys Winfred Thomas Agricultural Research Station located in Meridianville, AL in 2004-05 cropping season. Four winter canola cultivars (Banjo, Celsius, Explorer and Jetton) were planted in 100 x 50 m plots with 4 replications in randomized complete block experimental design. Conventional methods of field preparation were followed. Preplant herbicide (Treflan) was incorporated in the soil at the rate of 4.7 l/ha (2 qts/acre). Seeding was done with 18 -cm row spacing at 1.11kg/ ha (6 lbs/acre) seeding rate. Split applications of 13-13-13 NPK were made; 54 kg/ha was applied in the fall (6 weeks after planting) and 112 kg/ha in the spring. The crop was rain-fed and combine harvested at the end of the season.
Insect SamplingCollection of insects was done on canola at full bloom. Insects collected from thirty sweeps per plot using a standard size net comprised a sample. Gathered insects were placed into plastic bags and placed in a freezer overnight before insect count and identification were done. Canola pods sampled from 36 plants from 4 cultivars were examined for CSPW damage ( presence and number of exit holes). Only pods from the primary panicle per plant were examined. Table 1. Canola pods damaged by CSPW Fig. 12. Tarnished plant bugwww.uky.edu Fig. 11. Diamondback moth larvaFig. 5. Relative infestation levels (2002)Fig. 6. CSPW sweep counts (2002)Fig. 7. CSPW sweep count (2005)Fig. 8. DBM sweep count (2005)Fig. 9. TPB sweep count (2005)Fig. 10. CSPW, DBM and TPB on different cultivars (2005)LITERATURE CITEDDosdall, L. M. and L. S. Kott. 2006. Introgression of resistance to cabbage seedpod weevils to canola from yellow mustard. Crop Sci. 46 (2437-2445).Cebert, E., R. Ward and K. Ward. 2004. Canola yield deficit from pods damaged by cabbage seedpod weevil, Ceutorhynchus obstrictus (Marsham). Paper presentation. Entomological Society of America National Meeting, Salt Lake City, UT.
Rep(3 plants per rep)NumberPodsNo. Exit HolesNo. Damaged Pods% DamagedPods1231121342386553.7212831726450310846805648.1418184609049.7517230003017.4613932702627.47143541907351814558256544.8913162006247,310193956010147.71115975307849.61212874007457.8Total17486751022078444.8Damaged Pods (%)
CSPW SWEEP COUNT
BIONOmics Plot Yield June 9, 2006 Harvest; One passage of 6 rows? wide and 100 ft long
Plot #CultivarYield in Kg% Area Cover