Published in Agron J 99:842-846 (2007)
DOI: 10.2134/agronj2006.0236
© 2007 American Society of Agronomy
677 S. Segoe Rd., Madison, WI 53711 USA
Canola
Insect Pest Incidence and Injury to Herbicide-Tolerant Canola in Western Canada
H. A. Cárcamo* and
R. E. Blackshaw
Agriculture and Agri-Food Canada, Lethbridge Research Centre, Lethbridge, AB, Canada T1J 4B1
* Corresponding author (carcamoh{at}agr.gc.ca)
Received for publication August 21, 2006.
 |
ABSTRACT
|
|---|
Genetically modified herbicide-tolerant (HT) canola (Brassica napus L.) cultivars dominate the acreage planted to this oilseed crop in western Canada. We conducted a 3-yr (2000–2002) field study near Lethbridge, AB, Canada to compare the incidence and damage of three key insect pests in Roundup Ready, Liberty Link (both HT), and Q2 (a conventional cultivar). Flea beetle (Phyllotreta cruciferae and P. striolata [Coleoptera, Chrysomelidae]) damage in 2000 ranged from 31 to 32% of foliage consumption at the cotyledon stage, which surpassed economic thresholds but was not related to cultivar. Cabbage seedpod weevil (Ceutorhynchus obstrictus Marsham [Coleoptera, Curculionidae]) damage to canola pods ranged from 58 to 70 holes per 100 pods in 2001, and lygus plant bugs (Lygus spp. [Heteroptera, Miridae]) at the early pod stage reached densities of >37 individuals per 10 sweeps in 2000 and 2002. For both insect pests, their damage or abundance was similar among the conventional and HT cultivars. We conclude that, in the short term, the insect pests common in southern Alberta do not exhibit a preference to invade and damage the transgenic herbicide canola cultivars commonly planted in the prairies.
Abbreviations: HT, herbicide tolerant
|
INTRODUCTION
|
|---|
CANOLA, ALSO KNOWN AS OILSEED RAPE, is among the top cash crops in Canada with over 5 million ha planted every year and an estimated overall contribution of >$6 billion (Canadian dollars) to the national economy (Canola Council of Canada, 2005). More than 100 HT cultivars have been introduced during the last decade, and by 2005 they accounted for 95% of the total canola acreage (Canola Council of Canada, 2005). Transgenic cultivars tolerant to glyphosate (55%) and glufosinate (28%) are dominant. Clearfield varieties (BASF, Mississauga, ON, Canada) (tolerant to imazamox and imazethapyr), bred using traditional methods, accounts for 12% of canola production. The convenience of controlling a wide spectrum of weeds with a single postemergence application and increased yields and economic returns explain the rapid adoption of this technology (Harker et al., 2003). Massive adoption of new technologies in agriculture can affect agroecosystem components including existing or potential pests such as weeds, pathogens, and insects. Broadleaf, grassy, annual, and perennial weeds vary in their susceptibility to glyphosate and glufosinate; therefore, the diversity of weeds in crops can be affected by the choice of HT canola system (Harker et al., 2000), which in turn may influence communities of arthropods (Norris and Kogan, 2000).
The steady increase of canola acreage in the Canadian prairies has resulted in a concomitant increase in the incidence of insect pests (Lamb 1989). Plant bugs in the genus Lygus (Lygus elisus, L. keltoni, and L. borealis in southern Alberta) are native polyphagous insects that are chronic pests of seed alfalfa, but occasionally reach pest densities during the early pod stage and cause economic damage by feeding on the seeds of ripening canola pods (Butts and Lamb, 1990). A serious outbreak of lygus bugs throughout Alberta from 1996 to 1998 required spraying of about half a million hectares of canola. The most serious widespread pests of canola in the prairies are introduced flea beetles that feed on seedlings and can kill them at the cotyledon stage, thus reducing stand density and delaying maturity, which increases the risk of losses to frost in the fall (Lamb, 1984). In southern Alberta and Saskatchewan, cabbage seedpod weevil, another pest of European origin, has been increasing during the past 10 yr to become a serious threat to production, occasionally requiring insecticide sprays (Cárcamo et al., 2005).
Genetic variability among HT crop cultivars along with the associated chemical weed management system may affect their susceptibility to insect pests, but despite their wide adoption, they have not been assessed with respect to insect abundance and damage. Inherent variability in seedling vigor may influence tolerance to flea beetle damage and/or their ability to rapidly grow past the vulnerable cotyledon stage (Elliott et al., 2005). Other agronomic traits of the cultivars can influence insects. For example, cultivars that flower earlier can attract more cabbage seedpod weevils and suffer greater damage (Cárcamo et al., 2007). Indirect effects of HT canola on insects may result from increased incidence of grassy weeds in glufosinate systems that add vegetation diversity to the agroecosystem. In a study of transgenic and conventional soybeans, Buckelew et al. (2000) found that leaf hoppers (Empoasca fabae Harris) were more abundant in plots with fewer weeds, but the number of tarnished plant bugs (L. lineolaris) was higher in the weedier plots. Changes in weed communities of canola agroecosystems may indirectly increase the abundance of insect pests by providing alternate broadleaf hosts for lygus bugs (generalists), but reduce the populations of specialists, such as flea beetles and cabbage seedpod weevils that may be deterred by the presence of grasses and other nonhosts.
The objective of this study was to compare the incidence of three key insect pests and their injury: (i) flea beetles, (ii) cabbage seedpod weevil, and (iii) lygus bugs in conventional canola (Q2) managed with standard herbicides and two HT cultivars (Liberty Link and Roundup Ready) managed with their designated herbicides. These cultivars were selected because they were the most commonly grown at the time and would provide a realistic comparison of insect incidence and injury. Parental lines of HT cultivars were not available for comparison and furthermore, would not provide a relevant agronomic test since such genotypes are not planted commercially.
|
MATERIALS AND METHODS
|
|---|
The study site was located 6 km east of Lethbridge, AB (49°42.572' N; 112°41.386' W), in the dark-brown soil zone and mixed grass prairie ecoregion (Ecological Stratification Working Group, 1995). Three canola cultivars were selected to compare insect abundance and damage: (i) Liberty Link 2663 (glufosinate resistant), (ii) Roundup Ready LG 3235 (glyphosate resistant), and (iii) Q2, a conventional cultivar. Four replicate plots (6 x 10 m) per cultivar were planted in a randomized block design with fungicide (Thiram)-treated seed at 6.5 kg ha–1 on 23 May 2000, 20 Apr. 2001, and 10 May 2002. Standard agronomic recommendations for canola production were followed (Table 1) and plots were irrigated as needed. In 2001 a severe infestation of volunteer oat (Avena sativa L.) required additional spraying with sethoxydim in all plots (Table 1) and manual removal of broadleaf weeds, mainly thistles. Insecticides were applied only in 2002 on 7 and 26 June (Deltamethrin at 6.15 g ha–1) to manage a large outbreak of flea beetles.
View this table:
[in this window]
[in a new window]
|
Table 1. Chemical inputs applied to a herbicide-tolerant and conventional canola trial near Lethbridge, 2000–2002.
|
|
The three main insect pests of canola in southern Alberta were assessed for abundance or damage. At the seedling stage, flea beetle feeding was quantified by counting the number of plants with cotyledon damage in two transects of 20 seedlings per plot. A flea beetle damage index was calculated by classifying plants according to the percentage of the cotyledon consumed by flea beetles (0, <20, 20–40, 40–60, and >60). During flowering and at pod stages, abundance of lygus bugs and cabbage seedpod weevil were sampled by performing 10 walking sweeps (180° arc) per plot with a 38-cm-diam. sweep net. Upon crop maturity, when seeds began turning brown in the lowest pod of the primary raceme, five plants were collected at random from each plot in 2000 to assess larval exit holes made by the cabbage seedpod weevil. Twenty-five pods from the primary racemes of these plants were further examined under a dissecting stereoscope for a detailed analysis of feeding damage by adults of these three insects. In 2001 and 2002 near harvest time, cabbage seedpod weevil damage was assessed by randomly selecting 10 plants per plot and assessing all the pods from the primary, middle, and basal branch as recommended by Cárcamo et al. (2004). Weed abundance (broadleaf and grasses) and canola crop stages were quantified using two, 0.25-m2 quadrats per plot on 26 June 2000 and 27 Aug. 2001.
Estimates of insect damage expressed in proportions (flea beetle feeding on cotyledons and proportion of pods with weevil exit holes) were arc-sin transformed before one-way ANOVA. Lygus bug averages among plots were compared as total abundance, including all nymphal instars and adults. Furthermore, adult lygus bugs were separated into the three species that represented >99% of the lygus plant bug assemblage and analyzed using a factorial ANOVA with canola cultivar, lygus species, and sex as independent factors, and average abundance per sweep for the entire season catch as the dependant variable. A regression analysis was also conducted to assess the potential influence of insect abundances, including dates of collection, and damages on log-transformed (log + 1) seed yield. All statistical analyses were done with Systat for Windows Version 10.2 (Wilkinson, 2002).
|
RESULTS AND DISCUSSION
|
|---|
All three insect pests of canola selected for this study were found every year in our plots, often above economic thresholds, but their abundances or damage were similar in the conventional and transgenic cultivars every year (Table 2, for all comparisons, P > 0.05).
View this table:
[in this window]
[in a new window]
|
Table 2. Abundance of insect pests per 10 sweeps, their damage, weed abundance (per 0.25 m2), and seed yields in conventional (Q2) and herbicide-tolerant canolas [Liberty link (LL) and Roundup Ready (RR)].
|
|
Flea beetle damage to cotyledons averaged 
31% in 2000, which was above the nominal economic threshold of 25% used by producers in the area. This nominal threshold is supported by Gavloski and Lamb (2000), who demonstrated plant compensation from flea beetle feeding at this level. Because of the late seeding on 23 May 2000 and a delay in germination caused by spring drought, an unusual amount of insect damage was observed on canola pods, but this damage also was not influenced by canola variety. In 2001 cotyledon damage was 15%, but in 2002, flea beetles consumed >50% of the cotyledons regardless of cultivar (Table 2) and required insecticide treatment. The lower damage observed in 2001 resulted from the earlier seeding date (mid April) compared with the May seeding in the other years. Growers in southern Alberta traditionally plant their canola crops as early as possible, usually around the second or third week in April to escape flea beetle damage and utilize early season moisture from melting snow. This recommendation is opposite for other areas, such as North Dakota, USA, where later-seeded crops tend to suffer less damage (Milbrath and Weiss, 1995). Despite the substantial flea beetle damage observed in two of the years, regression analysis suggested no influence of this insect on canola yield in any of the years (t tests of regression coefficients, P > 0.05). This finding confirms other reports where seedlings have sustained 50% defoliation without a noticeable effect on yield (Nowatzki and Weiss, 1997). However, delayed maturity effects can leave the plants vulnerable to frost damage and other insect pests.
Cabbage seedpod weevil adults reached outbreak levels in southern Alberta in 2000 and 2001 (Cárcamo et al., 2005; Dosdall et al., 2006). However, in 2000, because of the delay in seeding, they averaged only two to five weevils per sample of 10 sweeps in our plots at early flower, which was well below the regional economic threshold of 30 to 40 weevils per sample (Table 2). At early pod in 2000, weevil numbers were slightly higher than at early flower, and ranged from 10 to 17 adults per sample. These weevils represented the new generation that overwinter without reproducing and normally do not cause economic damage. However, because of the late seeding in 2000, a large proportion of pods had punctures and seed damage caused by these adults. Similar to flea beetles, weevils fed on pods of all cultivars to the same extent (Table 2). Canola fields planted after the middle of May generally accumulate fewer weevils than fields planted in April (Dosdall and Cárcamo, 2001–2004, unpublished data). In 2001, plots were seeded on 19 April and accumulated 30 to 40 weevils per sample at early flower regardless of canola cultivar. In 2002, weevil abundances were four to seven weevils per sample as populations had declined as a result of consecutive droughts during the previous 2 yr. Damage caused by the cabbage seedpod weevil larvae (proportion of pods with exit holes, Table 2) followed adult abundance pattern and was statistically similar among cultivars although the conventional variety (Q2) had slightly lower damage than the transgenic cultivar Liberty Link in 2001 (58 vs. 70% damage). In 2001 76% of Liberty Link plants had reached the pod stage by 27 July in contrast to only 42% for the conventional Q2 variety. Cultivars that flower early are expected to sustain higher damage and this concept has been incorporated in a trap crop system for this pest (Cárcamo et al., 2007). Therefore, differences in cabbage seedpod weevil abundance and other insects are likely related to crop phenology and planting dates rather than genetic manipulations for herbicide resistance.
The Lygus plant bug species complex was dominated by L. elisus and L. keltoni. One or both of these species were significantly higher in number than L. borealis in every year of the study (Fig. 1; range of F values = 7.36–118.51, P < 0.01), but canola cultivar had no effect on any of the species, nor was there an interaction between cultivar and Lygus species (F2,4 = 0.17–2.34, P > 0.05; Fig. 2); the sex ratio was also similar across cultivars (data not shown). In 2000, overall lygus bug abundance, including first instars to adults, averaged >83 bugs per 10 sweeps at the critical early pod stage and was well over the economic thresholds of 15 to 20 bugs per sample of 10 sweeps (Table 2). Lygus abundance at early pod in 2000 was the only insect variable that was negatively related to canola yield (Fig. 2, R2 = –0.76, t = –5.67, P < 0.001); however, there was no significant difference in lygus bug abundance among the cultivars in 2000 (F2,9 = 0.662, P = 0.54). At the flower stage, lygus bugs were more abundant than at the early pod stage, yet their abundance was not related negatively to yield. This result agrees with those of Wise and Lamb (1998), where lygus bug abundance at the pod stage was more critical to canola yield than at the flower stage. At the flowering stage, the majority of lygus bugs in canola are represented by smaller juveniles (first to third instar) that may not damage the plants, or alternatively, the plants compensate for it (Jones et al., 2003). Overall lygus bug abundance in 2001 and 2002 was also above economic thresholds (>45 bugs per sample of 10 sweeps), but did not reduce yield and also did not differ among cultivars.
View larger version (19K):
[in this window]
[in a new window]
|
Fig. 1. Abundance of three lygus plant bug species (±SE) in canola planted near Lethbridge, AB, in 2000, 2001, and 2002.
|
|
View larger version (13K):
[in this window]
[in a new window]
|
Fig. 2. Lygus plant bugs at the early pod stage of canola in 2000 significantly reduced yield in three cultivars (pooled data, n = 12 plots) planted near Lethbridge, AB (R2 = –0.76, t = –5.67, P < 0.001).
|
|
Differences in weed abundance were significant only in 2000 (Table 2), when broadleaf weeds were significantly higher in number in the plots with the conventional Q2 cultivar than in plots with the Liberty Link cultivar (P < 0.05). Grassy weeds were very few in 2000, but in 2001 there were slightly more of these in the conventional plots than in those with the HT cultivars. Weed species and abundance data was collected from the same three cultivars in a long-term study (treated with insecticides) planted adjacent to this test. A preliminary summary of the weed data from that study for 2000–2004 suggested considerable variability in weed abundance among the treatments. In that study, graminaceous and broadlead weeds were similar in abundance per square meter (or even fewer in some years) in conventional plots than in those with HT cultivars (Blackshaw, 2007, unpublished data) most years. This variability results from the timing of rainfall and the germination of weeds after herbicide application in the HT crops where treatment was applied only at a specified crop stage to avoid crop injury. One of the factors expected to influence insect pest injury to crops was weed abundance (Buckelew et al., 2000); therefore, the apparent inconsistency of this variable in our study can explain the lack of differences in insect pest abundance and injury to the crops. Because most farmers are very efficient at weed control, even in conventional canola, it is unlikely that weeds will influence the composition, abundance, and damage risk from insect pests in this crop.
In conclusion, we found high abundance of insects and damage to canola at our study site; however, both variables were not affected by the type of canola planted. The overall abundance and damage varied from year to year, depending on how late the crop was seeded. In 2000, when the crop was planted late, cabbage seedpod weevils were not abundant at the critical early flower stage, but lygus bugs clearly surpassed economic thresholds and reduced crop yields in all cultivars. In 2001 the crop was seeded earlier and it escaped flea beetles, but attracted large numbers of cabbage seedpod weevil. Phenology differences among cultivars, regardless of whether they were transgenics or conventional, were likely more important in determining insect abundance and potential economic damage. We conclude that, in the short term, the insect pests common in southern Alberta do not exhibit a preference to invade and damage the transgenic HT canola cultivars commonly planted in the prairies. Potential long-term landscape level effects on arthropods, including natural enemies of insect pests, resulting from changes in plant communities in ecosystems with HT crops remain to be studied.
|
ACKNOWLEDGMENTS
|
|---|
We thank B. Postman and C. Herle for technical support, and J. Yanish and E. Cadieu for text and graphic assistance, respectively.
|
NOTES
|
|---|
This study was financed through the Agriculture and Agri-Food Canada A-base research program.
|
REFERENCES
|
|---|
- Buckelew, L.D., L.P. Pedigo, H.M. Mero, M.D.K. Owen, and G.L. Tylka. 2000. Effects of weed management systems on canopy insects in herbicide-resistant soybeans. J. Econ. Entomol. 93:1437–1443.[Web of Science][Medline]
- Butts, R.A., and R.J. Lamb. 1990. Injury to oilseed rape caused by mirid bugs (Lygus) (Heteroptera: Miridae) and its effect on seed production. Ann. Appl. Biol. 117:253–266.[Web of Science]
- Canola Council of Canada. 2005. Available at www.canola-council.org/ind_overview.html [verified 8 Mar. 2007]. Canola Council of Canada, Winnipeg, MB.
- Cárcamo, H.A., L.M. Dosdall, D. Johnson, and O. Olfert. 2005. Evaluation of foliar and seed-coated insecticides for control of the cabbage seedpod weevil (Coleoptera: Curculionidae) in canola. Can. Entomol. 137:476–487.
- Cárcamo, H.A., R. Dunn, L.M. Dosdall, and O. Olfert. 2007. Managing cabbage seedpod weevil in canola using a trap crop—A commercial scale study in western Canada. Crop Prot. (in press, available online 16 Jan. 2007).
- Cárcamo, H.A., T. Entz, and R.E. Blackshaw. 2004. Sub-sampling canola plants (Brassica napus L) to estimate cabbage seedpod weevil (Ceutorhynchus obstrictus Marsham) damage. J. Entomol. Sci. 39:122–124.
- Dosdall, L.M., B.J. Ulmer, G.A.P. Gibson, and H.A. Cárcamo. 2006. The spatio-temporal distribution dynamics of the cabbage seedpod weevil, Ceutorhynchus obstrictus (Marsham) (Coleoptera: Curculionidae), and its larval ectoparasitoids in canola in western Canada. Biocont. Sci. Technol. 16:987–1006.
- Ecological Stratification Working Group. 1995. A national ecological framework for Canada. Agriculture and Agri-Food Canada, Research Branch, Centre for Land and Biological Resources Research and Environment Canada, State of the Environment Directorate, Ecozone Analysis Branch, Ottawa.
- Elliott, R.H., L.W. Mann, and O. Olfert. 2005. Vigor tests for evaluating the performance of Argentine canola (Brassica napus L.) under different growing conditions. Seed Technol. 27:273–285.
- Gavloski, J.E., and R.J. Lamb. 2000. Compensation for herbivory in cruciferous plants: Specific responses to three defoliating insects. Environ. Entomol. 29:1258–1267.[Web of Science]
- Harker, K.N., R.E. Blackshaw, K.J. Kirkland, D.A. Derksen, and D. Wall. 2000. Herbicide-tolerant canola: Weed control and yield comparisons in western Canada. Can. J. Plant Sci. 80:647–654.
- Harker, K.N., G.W. Clayton, R.E. Blackshaw, J.T. O'Donovan, and F.C. Stevenson. 2003. Seeding rate, herbicide timing and competitive hybrids contribute to integrated weed management in canola (Brassica napus). Can. J. Plant Sci. 83:433–440.
- Jones, J.W., H.A. Cárcamo, J.K. Otani, R.A. Butts, R.H. McKenzie, E.D. Solberg, and J. DeMulder. 2003. Does canola compensate for lygus bug damage? Final Rep. 1999M462. Canola Council of Canada and Alberta Agricultural Research Institute, Edmonton, AB, Canada.
- Lamb, R.J. 1984. Effects of flea beetle, Phyllotreta spp. (Coleoptera: Chrysomelidae) on the survival, growth, seed yield and quality of canola, rape and yellow mustard. Can. Entomol. 116:269–280.
- Lamb, R.J. 1989. Entomology of oilseed Brassica crops. Annu. Rev. Entomol. 34:211–229.[Web of Science]
- Milbrath, L.R., and M.J. Weiss. 1995. Influence of tillage system, planting date, and oilseed crucifers on flea beetle populations (Coleoptera: Chrysomelidae). Can. Entomol. 127:289–293.
- Norris, R.F., and M. Kogan. 2000. Interactions between weeds, arthropod pests, and their natural enemies in managed ecosystems. Weed Sci. 48:94–158.
- Nowatzki, T., and M.J. Weiss. 1997. Effects of simulated and flea beetle injury to cotyledons on growth of drought-stressed oilseed rape, Brassica napus. Can. J. Plant Sci. 77:475–481.
- Wilkinson, L. 2002. Systat Software. v. 10.2. Systat Software, Richmond, CA.
- Wise, I.L., and R.J. Lamb. 1998. Economic threshold for plant bugs, Lygus spp. (Heteroptera: Miridae), in canola. Can. Entomol. 130:825–836.
TOP
ABSTRACT
INTRODUCTION
