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a Dep. of Agricultural, Food and Nutritional Science, 4-10 Agriculture/Forestry Centre, Univ. of Alberta, Edmonton, AB, Canada T6G 2P5
b Dep. of Plant Agriculture, Univ. of Guelph, Guelph, ON, Canada N1G 2W1
* Corresponding author (lloyd.dosdall{at}ualberta.ca)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The cabbage seedpod weevil is univoltine. Sexually immature adults overwinter beneath the soil primarily in tree shelterbelts, and in spring they are attracted to canola crops in the bud to early flowering stages (Dosdall and Moisey, 2004). When plants are still flowering but siliques on lower racemes are enlarging, gravid females can excavate a cavity in an immature pod with their mouthparts and may deposit a white, cylindrical egg into the puncture. Larvae feed on developing seeds within the pods, and each larva consumes about five to six seeds during development that spans about 20 to 43 d in southern Alberta (Dmoch, 1965; Dosdall and Moisey, 2004). Mature larvae chew circular exit holes in the walls of the siliques, drop to the soil surface, burrow in, and pupate. New generation adults emerge approximately 14 d later and feed on brassicaceous plants to build up fat stores for overwintering. Late in the season when temperatures decline, weevils migrate to shelterbelts where they remain in diapause until soil temperatures warm to approximately 15°C in spring (Bonnemaison, 1957; Dmoch, 1965; Ni et al., 1990; Ulmer and Dosdall, 2006b).
Ceutorhynchus obstrictus can cause economic losses to canola at several stages of crop development; consequently, the species poses a serious threat to canola production in the Northern Great Plains. When overwintered adults invade crops, primarily in the bud to early-flowering stages, they feed on flower buds often destroying their vascular tissue and causing buds to desiccate ("bud-blasting") (Dosdall et al., 2001). Plants with severe bud-blasting produce racemes with few pods, and plants may even fail to flower when weevil densities are high. Feeding by larvae within pods on developing seeds is a major source of yield loss (Free et al., 1983; Buntin, 1999). If environmental conditions are humid after larvae bore exit holes, the pods can be invaded by fungal spores that germinate and destroy additional seeds within the pods (Dosdall et al., 2001). New generation adults emerge late in the season and feed on seeds within green pods to build up fat stores for overwintering, causing reductions in yield and crop quality (Buntin et al., 1995).
Transfer of potentially useful traits among different crops has often been limited by species barriers (Ripley and Arnison, 1990). Yellow mustard was previously reported to be resistant to C. obstrictus (Doucette, 1947; Harmon and McCaffrey, 1997; Brown et al., 1999; Kalischuk and Dosdall, 2004), and evidently carries genes for resistance. Embryo rescue has been used to overcome species barriers in difficult crosses among Brassicaceae, for example, between B. rapa and B. oleracea L. (Harberd, 1969; Snell, 1978), and B. napus and B. oleracea (Ayotte et al., 1987). The technique involves "rescuing" embryos by excising them from the ovaries or ovules before they are aborted by the plant and then culturing them in vitro on artificial media, or occasionally in vivo (Sharma et al., 1996).
At present, the only control strategy available to canola producers is application of broad-spectrum chemical insecticides (Dosdall et al., 2001). However, development of canola varieties resistant to the weevil would be a more environmentally sustainable control strategy. Introgression of genes for resistance to C. obstrictus from resistant (S. alba) to susceptible (B. napus) genotypes through embryo rescue has much potential for developing canola cultivars resistant to this pest. Promising results have been achieved using this approach for developing resistance to other insect pests of canola, particularly the cabbage maggot, Delia radicum (L.), and the turnip maggot, D. floralis (Fallén) (Diptera: Anthomyiidae) (Dosdall et al., 2000; Kott and Dosdall, 2004; Ekuere et al., 2005). In this study we evaluated S. alba x B. napus hybrid genotypes to determine their relative resistance levels to C. obstrictus. Mechanisms of resistance were also investigated in genotypes that demonstrated resistance in field and laboratory trials.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The strategy for identifying intergeneric germplasm with genes for resistance to the cabbage seedpod weevil and determining the mechanism(s) of resistance followed a series of steps that first involved field-testing the 230 doubled haploid genotypes for susceptibility to infestation by C. obstrictus. Because only small quantities of seed were available in the early stages of the screening process, this initially involved nonreplicated assessments with intergeneric genotypes interspersed with frequent plantings of a B. napus check line known to be susceptible to the weevil. The most promising lines were then retested, again in nonreplicated studies due to low seed availability, and seed of selections demonstrating evidence of weevil resistance in the two field tests was then multiplied in the greenhouse. Field assessments proceeded with replicated trials of seven promising genotypes compared with the B. napus check line. Replicated field assessments were complemented with replicated laboratory tests, where excised pods of uniform sizes were exposed to mated, gravid females; pods were then dissected for the presence of eggs. Finally, studies on mechanisms of resistance were performed by examining larval developmental parameters in plants of susceptible and resistant genotypes and comparing weevil ovipositional behaviors in the different genotypes. A summary of the trials conducted each year is presented in Table 1.
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Field Plot Experiments
In 2001, a field study was conducted on an irrigated site approximately 10 km east of Lethbridge, AB (49°37' N, 112°39' W), to assess the susceptibilities of 230 intergeneric lines to attack by C. obstrictus. Due to limitations in quantities of seed available, each genotype was seeded on 14 May 2001 in a single 6-m row, at a density of approximately 300 plants per row. Soil was fertilized according to the soil test recommendations for canola production and seeding was performed into cereal stubble with a coulter double disc no-till drill. The single-row plots were interspersed every fifth row with B. napus cv. Q2, a variety of B. napus known from previous research to be comparatively susceptible to cabbage seedpod weevil infestation (Dosdall, 2001). All seed was treated with Vitavax rs (22.5 mL kg–1 containing carbathiin/thiram/lindane at 1:2:15 g a.i.) to provide seedling protection against herbivory by flea beetles (Phyllotreta spp.) (Coleoptera: Chrysomelidae). At the end of the season, after all larval development had occurred within infested pods, 35 plants were selected randomly from each plot, bagged, and labeled. The numbers of pods per plant and the numbers of pods with weevil exit holes were counted and recorded.
The genotypes having low susceptibility to infestation by C. obstrictus in 2001, determined as having less than or equal to a mean of 0.05 exit holes per pod, were grown in greenhouse chambers in 2002 for seed multiplication. Eighteen hybrids, in addition to three intergeneric hybrids not evaluated in 2001 due to low seed availability, were planted in single-row field plots on 16 May 2002 for retesting according to the field protocol described above. Soil was fertilized according to the soil test recommendations for canola production and seeding was performed into cereal stubble with a coulter double disc no-till drill. At the end of the season, after all cabbage seedpod weevil development was completed within infested pods, 35 plants were selected randomly from each plot, bagged, and labeled. The numbers of pods per plant and the numbers of pods with weevil exit holes were counted and recorded.
In 2003, field evaluations of germplasm for susceptibility to infestation by C. obstrictus could not be undertaken due to very low weevil populations in southern Alberta. However, seed increases were conducted in 2003 in greenhouse chambers.
In 2004, field assessments to validate resistance to C. obstrictus were performed near Lethbridge with selected genotypes in replicated trials conducted at two sites located approximately 5 km apart. Seven genotypes were selected for field testing based on quantities of seed available and low susceptibilities to cabbage seedpod weevil infestation in previous field and laboratory arena studies (Genotypes 83, 128, 305, 329, 362, 394, and 428). The weevil-susceptible B. napus cultivar, Q2, was also included in the study for comparison. Genotypes were sown into cereal stubble on 26 April with a coulter double disc no-till drill in a randomized complete-block experimental design. Within each block, each line occupied a single 6-m row, seeded at a density of approximately 300 plants per row. Plots were replicated four times at each site. Susceptibility assessments were performed at the end of the season according to the method specified previously from 10 plants collected randomly from each treatment plot. Plants sampled were fewer than in preceding years (35 plants per plot in 2001 and 2002 versus 10 in 2004 and 2005) because research by Cárcamo et al. (2004) determined that 10 plants per treatment were sufficient to give a valid estimate of cabbage seedpod weevil infestation levels.
In 2005, a replicated field trial was performed near Lethbridge using B. napus cv. Q2, S. alba cv. Pennant, and the same seven intergeneric lines evaluated in 2004. Plots of each genotype comprising four rows spaced 20 cm apart were sown into cereal stubble on 30 April with a coulter double disc no-till drill in a randomized complete-block experimental design with four replications. Each 6-m row had a density of approximately 300 plants. Susceptibility to infestation by C. obstrictus was assessed at the end of the season according to the method specified previously from 10 plants collected randomly from each treatment plot (Cárcamo et al., 2004).
Laboratory Arena Trials
Laboratory assessments for the different genotypes and years of study are summarized in Table 1. In 2003, laboratory bioassays were used to assess possible antixenotic resistance of the most resistant genotypes selected during field evaluations conducted in 2001 and 2002. Ten hybrid genotypes found to have low susceptibility to infestation by C. obstrictus in field trials conducted in 2001 and 2002 were grown in greenhouse chambers in 2003 for seed multiplication. Seeds of each genotype were then sown in pots containing a mixture of soil, sand, peat, and a commercial potting mix, and maintained in a greenhouse at approximately 20°C, 14 h L/10 h D. Approximately 8 wk after planting, pods 40 to 60 mm long were excised from the plants and used to assess susceptibilities to ovipositing females of C. obstrictus.
Adults of C. obstrictus were collected from commercial canola fields for use in the laboratory bioassays. Weevils were maintained for no more than 2 d on a diet of 10% sucrose solution and B. napus flowers at room temperature. Females were separated using a binocular microscope, and pods of B. napus were used to confirm their ovipositional status before the experiments (Harmon and McCaffrey, 1997).
The experimental design was similar to that of Harmon and McCaffrey (1997). Replicate cages (18.5 by 18.5 by 8 cm high) held 121 evenly spaced pods of 11 genotypes per cage as a Latin square design of 11 rows and 11 columns. The genotypes assessed comprised 10 intergeneric lines plus B. napus cv. Q2. All tests used a pod/weevil ratio of 4:1. At initiation of the experiment, the females were dropped into the center of the arena. After 24 h, pods were removed, microscopically dissected, and the numbers of weevil eggs per pod were counted and recorded for each genotype.
In 2004, two sets of laboratory arena trials were conducted to assess susceptibilities of genotypes to C. obstrictus infestation under controlled environment conditions. Two trials were conducted because pods of the different genotypes of standard developmental stages were not all available at the same time. In Trial 1, susceptibilities of six lines were compared with that of B. napus cv. Q2, comprising Genotypes 22, 128, 305, 362, 394, and 428. Five lines, comprising Genotypes 83, 329, 408, 537, and 578 were evaluated in comparison with B. napus cv. Q2 in Trial 2. The method used was the same as described above, except that the design involved seven rows and seven columns (Trial 1) and six rows and six columns (Trial 2). Trials 1 and 2 were each repeated five times, each with a pod/weevil ratio of 4:1. Egg numbers and weevil feeding marks per pod were counted after 24 h (Harmon and McCaffrey, 1997; Kalischuk and Dosdall, 2004).
Larval Growth and Development
In 2005, five seeds of each of nine lines (Genotypes 22, 83, 128, 151, 329, 360, 374, 394, and 428) were planted (one part cornel, one part washed sand, and one part soil composed of three parts soil, one part peat, and one part sand) and plants were maintained in a greenhouse at approximately 20°C, 14 h L/10 h D. Approximately 8 wk after planting, pods ranging in length from 40 to 60 mm were abundant on the plants, and each plant was exposed to 300 (approx. 1:1, male/female) field-collected weevils for 24 h. The weevils were confined to the plants using fine mesh bags (2 m long with a 0.75-m diameter, and 1-mm mesh), with both ends of the bags tied to prevent their escape. Plants were placed outdoors in partial sunlight and bottom-watered while in the cages.
The mesh bags and adult weevils were removed after exposure for 24 h and plants were placed in a growth chamber set at 22 ± 1°C, 16 h L/18 ± 1°C, 8 h D. Plants were bottom-watered daily. Seven to ten days after infestation, cardboard trays were fixed to the potted plants to catch weevil larvae as they dropped from the pods. The trays were fitted around the stems of the plants and placed on top of the pots. The trays were 3600 cm2 with their edges raised on all sides to a height of 8 to 10 cm. Weevil larvae were collected daily, immediately frozen, and later dried at 60°C for 48 h before being weighed. Developmental times in days, from egg to emergence of final-instar larvae from pods, were recorded for each genotype evaluated.
Adult Oviposition Behavior
In 2005, plants of B. napus cv. Q2, and intergeneric lines 22, 83, 128, 151, 360, 374, 394, and 428 were grown in the greenhouse as described above. Mated, egg-laying females were obtained as described above in the laboratory arena trials.
Oviposition cages were 65-mL clear plastic cylindrical vials (79-mm length, 33-mm diameter). A hole just large enough to insert the peduncle of a pod was drilled in the bottom of each cage. A large hole was cut in the lid and covered with 1-mm-diam. mesh to allow for air flow within the cage. Individual cages were mounted within an 18-cm3 cardboard box, open in the front for observation, but closed on all other sides.
For each replicate oviposition trial, a pod of one of the genotypes was secured to the bottom of a cage by pulling the peduncle through the drilled hole. A single mated, gravid female was added, and the cage was mounted horizontally on the stage. The start time was recorded for each trial. Each female was given a base time of 30 min, and if no oviposition behaviors occurred during that time, it was replaced with a different specimen. Oviposition events were distinguished according to the sequence described by Kozlowski et al. (1983), and included: pod exploration, egg cavity formation, turn, egg deposition, ovipositor retraction, pod brushing, and pod abandonment. For this study, time measurements for each of the above behaviors were recorded with a stop watch, although the turn and ovipositor retraction steps were considered the beginning and end of the egg deposition behavior, respectively. Females that did not complete the entire oviposition sequence were not included in the analysis. Weevils and pods used in each trial were never used in subsequent trials. Pods in which a weevil had successfully oviposited were dissected microscopically to confirm the presence of an egg. The procedure was repeated until data for 15 ovipositing females were recorded per genotype.
Data Analyses
Significance of differences in numbers of weevil exit holes per pod, eggs laid per pod, and feeding marks per pod among the different hybrid genotypes were determined for the replicated field trials and the laboratory arena trials using analysis of variance (ANOVA) and Tukey's studentized range test (P < 0.05) (SAS Institute, 1999). Transformations of the data [log10(x + 1)] were performed as necessary to meet the assumption of normality when preliminary frequency analyses indicated that the data were not normally distributed. Because each arena trial was repeated several times, the analyses were performed by pooling data from each of the separate trials. Larval weights and developmental times from the no choice test were subjected to ANOVA and the means were compared with Tukey's test (P < 0.05) (SAS Institute, 1999). Data for each component of the oviposition behavior sequence, as well as the total time from beginning to end of oviposition, were subjected to ANOVA and the means were compared with Tukey's test after performing log10(x+1) transformations (P < 0.05) (SAS Institute, 1999).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Laboratory Arena Trials
Genotypes determined in the 2002 field evaluation to have less than or equal to 0.05 exit holes per pod, and evaluated in laboratory arenas in 2003, varied in their susceptibilities to oviposition by female weevils (Table 3). Genotype 374 and B. napus cv. Q2 had significantly more eggs deposited per pod than all other lines except Genotypes 305 and 408 (P < 0.05). Genotypes 394, 329, 83, and 22 had significantly fewer eggs per pod than Genotypes 374, 408, and B. napus cv. Q2 (P < 0.05) (Table 3).
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In the laboratory arena trials conducted in 2004 to compare susceptibilities of five intergeneric hybrid genotypes with B. napus cv. Q2, mean feeding marks per pod were significantly higher in plants of Genotypes 537 and 578 than in Genotypes 408 and 83. Mean eggs per pod were significantly higher in plants of Genotype 537 than in plants of Genotype 83 (P < 0.05), and the remaining genotypes, including B. napus cv. Q2, were intermediate in their susceptibilities to C. obstrictus infestation (Table 3).
Larval Growth and Development
When confined on plants of S. alba cv. Pennant in a no-choice situation, C. obstrictus larval development time from egg to completion of the final larval instar was approximately 24% longer than on plants of B. napus cv. Q2 (P < 0.05) (Table 4). Among the nine hybrids evaluated, larval developmental time on Genotype 428 was significantly longer than on Genotypes 128, 329, 360, 374, and 394. Among the hybrid genotypes, most rapid pre-imaginal development occurred in pods of Genotypes 128, 329, 360, 374, and 394 (Table 4).
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Oviposition Behavior
No significant differences were observed in the times taken for pod exploration, egg cavity formation, or egg deposition between B. napus and the different intergeneric genotypes tested (P > 0.05) (Fig. 3). However, females required significantly more time for pod brushing on Genotype 394 than on the other intergeneric genotypes evaluated (P < 0.05). Total time from initiation of pod exploration to completion of pod brushing was significantly faster on Genotype 22 than on all other genotypes evaluated (P < 0.05) (Fig. 3).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Although other researchers have attempted to introgress genes for resistance to C. obstrictus from S. alba to B. napus, this study is the first to document evidence of weevil resistance in hybrid genotypes. For example, Kalischuk and Dosdall (2004) used both field and laboratory assessments to compare susceptibilities of four intergeneric genotypes with seven Brassicaceae species, and found susceptibilities of the hybrids to be similar to B. napus. McCaffrey et al. (1999) evaluated a single S. alba x B. napus intergeneric genotype and found that weevil resistance was not conferred to the hybrid. However, even though McCaffrey et al. (1999) found high egg deposition in pods of the hybrid, fewer larvae of C. obstrictus developed to maturity within the pods, suggesting some expression of antibiosis resistance in the hybrid. Our discovery of several genotypes that express varying levels of resistance to infestation by C. obstrictus can be attributed to the large number of intergeneric genotypes that were assessed in this study. Our sample of 230 doubled haploids was substantially greater than those of earlier researchers, and so involved higher probability of demonstrating successfully introgressed genes of interest.
Sinapis alba and some intergeneric genotypes appeared to exhibit both antibiotic and antixenotic resistance to C. obstrictus according to the definitions of Painter (1951) and Kogan and Ortman (1978). In the no-choice experiment, larval development of C. obstrictus was approximately 24% more rapid on B. napus cv. Q2 than on S. alba (Table 4), indicating antibiotic resistance in S. alba. Antixenotic resistance also characterized S. alba because Kalischuk and Dosdall (2004) and Ulmer and Dosdall (2006a) found that mated, gravid females only rarely oviposited in S. alba pods. Levels of resistance were lower in the intergeneric genotypes, but still demonstrated both resistance types. For example, although mean larval dry weights of the genotypes did not differ significantly from those of B. napus, larval developmental times were significantly slower in several hybrids than in B. napus (Table 4), indicating an antibiotic effect. Antixenotic resistance in the genotypes was evident in some laboratory arena tests, where mean eggs per pod were significantly higher in B. napus than in some hybrid genotypes (Table 3).
Ulmer and Dosdall (2006a) conducted ovipositional behavioral analyses of C. obstrictus on pods of several different Brassicaceae species and observed greater differences in discrete behaviors between species than those found in the current study where we investigated differences within one species (B. napus). Here we found that the time required for pod exploration, cavity formation, egg deposition, and pod brushing varied little between the hybrid genotypes and the susceptible B. napus check, providing little insight into the mechanism of resistance in the intergeneric genotypes. Ovipositional behavioral analyses should perhaps be replaced with olfactometry analyses in future studies of resistance mechanisms, to better understand the chemical signals that elicit responses by female weevils in selecting pods for oviposition.
Glucosinolate content and profile may be important in governing host plant resistance to the cabbage seedpod weevil. Adults respond to 3-butenyl and 4-pentenyl glucosinolate, and these compounds may influence acceptance of host plants by C. obstrictus (Evans and Allen-Williams, 1992; Bartlet et al., 1993; Blight et al., 1995; McCaffrey et al., 1999). McCaffrey et al. (1999, 2004) suggested that resistance in S. alba may be due to the high levels of sinalbin, p-hydroxybenzyl glucosinolate, and a lack of stimulatory kairomones such as 3-butenyl and 4-pentenyl glucosinolate. Glucosinolate analyses were not performed in the present study, but future research will involve comparative analyses of these compounds in different genotypes and in different plant structures to determine whether differences in susceptibility to infestation by the weevil can be explained by biochemical distinctions among the genotypes.
Susceptibility test results using laboratory arena trials did not always concur with results obtained in replicated field trials. For example, in the replicated field studies we determined that Genotypes 22, 83, 394, and 428 were significantly less susceptible to C. obstrictus infestation than B. napus cv. Q2 in terms of mean weevil exit holes per pod (Table 2), and these results were validated in arena trials conducted in 2003, but not in 2004 (Table 3). The reasons for these differences are not known, but this could be due to an inappropriate pod/weevil ratio used in the test arenas. We used a pod/weevil ratio of 4:1, as suggested by Harmon and McCaffrey (1997), but in 2004 very high numbers of eggs were deposited in the pods being tested. In 2003, only 27% of genotypes had, on average, more than two eggs deposited per pod, but in 2004, 77% of genotypes had a mean greater than two eggs per pod during the 24-h test period. In natural systems, more than one egg per pod can develop in canola, but this does not occur frequently because female weevils deposit a pheromone on pods after egg-laying, during abdominal brushing, that temporarily deters other females from ovipositing on those pods (Ferguson and Williams, 1991). The high frequency of multiple ovipositions in individual pods in 2004 suggests that weevils could exhibit little choice in selecting the most susceptible pods. We therefore recommend that future arena susceptibility tests be conducted with a higher pod/weevil ratio, perhaps 6:1 or 8:1, to provide a finer level of discrimination in detecting susceptible versus resistant germplasm.
In laboratory arena trials, data on numbers of feeding punctures per pod can be complementary to numbers of eggs laid per pod as measures of germplasm susceptibility to infestation by C. obstrictus (Kalischuk and Dosdall, 2004). However, in our 2004 laboratory arena trials we found that feeding punctures did not vary greatly among the resistant genotypes and the B. napus check (Table 3), which also suggests that pod/weevil ratios should be increased in future studies when assessing susceptibility of germplasm to the cabbage seedpod weevil.
Replicated field studies were perhaps the best measure of susceptibility to infestation of host plants by insects because they involved assessments under natural environmental conditions, where insects could exhibit preferences among different genotypes and experimental error could be estimated through replication and experimental design. Our replicated field trials determined that resistance levels among the intergeneric genotypes ranged from 2.5- to 8.3-fold greater than those of a standard commercial cultivar of B. napus (Table 2). Resistance levels were relatively consistent among the trials, and the resistance trait has evidently remained relatively stable in the plant population because it has been demonstrated over a number of generations. If the trait can be transferred successfully to elite canola cultivars, it is expected that these levels of resistance will be sufficient to maintain crop damage below the threshold at which insecticidal intervention is warranted, considering that canola can tolerate pod infestations of approximately 26% without measurable yield loss (Buntin, 1999; Free et al., 1983).
Our research to introgress genes for resistance to C. obstrictus from S. alba follows a similar initiative to develop germplasm resistant to the root maggots, D. radicum and D. floralis (Kott and Dosdall, 2004; Ekuere et al., 2005). We are following a similar breeding sequence involving production of many doubled haploids from crosses of S. alba x B. napus and then backcrossing progeny to the B. napus parent, selecting genotypes of appropriate quality in terms of seed glucosinolate content, selecting for insect resistance, performing crosses with elite canola breeding lines, confirming resistant phenotypes, and conducting yield trials to confirm agronomic performance. A next step will be to develop a linkage map using restriction fragment length polymorphisms from the original doubled haploids used in this study to identify quantitative trait loci (QTL) associated with weevil resistance. Our goal will be to identify and validate polymorphic markers linked to a weevil resistance locus or weevil resistance QTL to facilitate rapid transfer of genes for weevil resistance to commercial cultivars of canola using marker-assisted selection.
Development and registration of commercial cultivars of B. napus resistant to the cabbage seedpod weevil will have important benefits for the integrated management of this pest. The only control strategy currently available to canola producers is application of broad-spectrum pyrethroid insecticide, applied at 10% flower when populations exceed the economic threshold of three to four adult weevils per 180° sweep net sample (Dosdall et al., 2001). Such applications can be effective for reducing weevil populations, but are damaging to nontarget arthropods and beneficial species (Cárcamo et al., 2005). The level of resistance achieved through our breeding program should preclude the need for such applications, considering that canola can compensate for low to moderate levels of weevil damage (Lerin, 1984; Buntin, 1999). The longevity of the resistant germplasm can be extended if it is utilized as a component of an integrated management strategy for the cabbage seedpod weevil, rather than as a control mechanism used in isolation. Combining germplasm resistance with cultural and biological control tactics, like slightly delaying seeding date (Dosdall and Moisey, 2006), using recommended seeding rates (Dosdall and Moisey, 2006), and introducing host-specific natural enemies from the area of origin of the weevil (Kuhlmann et al., 2006), should help ensure that economic losses from this pest are greatly reduced and that this level of control can be sustained.
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ACKNOWLEDGMENTS |
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Received for publication March 1, 2006.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Journal of Environmental Quality |
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5% of pods with larval exit holes) and were excluded from further testing. White histograms were genotypes considered to have low susceptibility to C. obstrictus infestation (<5% of pods with larval exit holes) and were selected for further evaluations. The dotted line indicates the arbitrary cutoff between susceptible and resistant genotypes.