CABBAGE SEEDPOD WEEVIL CONTROL AND DAMAGE LOSS ASSESSMENT IN WINTER CANOLA USING INSECTICIDES

G. David Buntin

Department of Entomology, University of Georgia, Georgia Station, Griffin, GA 30223-1797 USA

ABSTRACT

Experiments examining the efficacy, timing and number of applications of various insecticides were used to assess cabbage seedpod weevil, Ceutorhynchus assimilis Paykull, yield loss relationships in winter canola, Brassica napus L.  Typically, the pyrethroid insecticides were more effective than currently registered insecticides endosulfan and methyl parathion at reducing adult numbers and preventing pod infestation by larvae.  Two insecticide applications during flowering usually were needed to effectively reduce adult numbers and prevented seed injury.  Larval injury primarily affected grain weight but did not consistently affect kernel weight or grain oil content.  Yield loss increased linearly by about 1.7% for each 1% increase in percentage of infested pods, when larval infestation of pods exceeded 22% infested pods.  These results support findings from Europe that canola can tolerate pod infestations of <26% without measurable yield loss.  Economic injury levels for varying control costs and commodity values ranged from 26 to 40% infested pods. These results provide a quantitative basis for the development of decision rules for C. assimilis which will minimize unnecessary insecticide use on canola in the United States.

KEYWORDS  cabbage seedpod weevil, Ceutorhynchus assimilis, canola, oilseed rape, Brassica napus, yield loss   

INTRODUCTION

            Cabbage seedpod weevil, Ceutorhynchus assimilis Paykull, is one of the most important insect pests of winter-grown oilseed rape and canola, Brassica napus L., throughout most of North America and Europe (Dmoch 1965, Free and Williams 1978, McCaffrey et al. 1986, Buntin and Raymer 1994) and of spring planted canola in the northwestern USA (Homan and McCaffrey 1993).  Ceutorhynchus assimilis has one generation per year and larvae feed on seed inside seedpods (Dmoch 1965).  Studies assessing yield loss caused by C. assimilis in Europe have found that each larva eats about 5 to 6 seeds (Lerin 1984) which reduces individual pod seed weight by about 18% (Free and Williams 1978, Free et al. 1983).  However, C. assimilis yield loss assessment in Europe is complicated by secondary attack of damaged pods by the pod midge Dasineura brassicae (Winnertz), a species that does not occur in North America.  The midge oviposit mainly through C. assimilis feeding or ovipositional wounds on pods (Free et al. 1983), where midge larvae can completely destroy remaining seeds in a C. assimilis infested pod.   In European studies in the absence of D. brassicae, canola seed yield is not measurably reduced by C. assimilis pod infestations of <26% infested pods (Lerin 1984, Free and Williams 1978).  The relationship between pod infestation and canola yield has not been assessed in North America.

            In Europe, pyrethroid insecticides including deltamethrin and alphacypermetrin are used to control C. assimilis (Garthwaite et al. 1995).  Pyrethroid insecticides are applied during flowering to kill adults before oviposition occurs and have less adverse impact on C. assimilis parasitoids and canola pollinators than organophosphate insecticides (Murchie et al. 1997).  Currently, C. assimilis management in canola in the USA consists of the prophylactic use and proper timing of the insecticides ethyl parathion, methyl parathion, or endosulfan (McCaffrey et al. 1986).  No other insecticides currently are registered for control of C. assimilis in oilseed rape in the USA.

            Selected foliar applied insecticides were tested for managing C. assimilis in  autumn-planted canola.  Additionally, the timing and number of applications needed during flowering for control were examined.  Results of these trials were used to assess yield loss relationships and develop economic injury levels for C. assimilis pod injury in canola.   

MATERIALS AND METHODS

Insecticide trials  Two trials were conducted in each year near Griffin, Ga., USA.  ‘Cobra’ canola was grown using conventional tillage and standard agronomic practices for winter canola. Plots measured 9.1 m x 9.1 m.  The first trial examined the efficacy of various insecticide treatments applied during bloom for C. assimilis larval control (Buntin 1999).  Treatments were applied when 50, 50 and 70% of plants had begun to bloom about 1 April in 1992, 1993, and 1994, respectively.  A second application was made to all plots 12, 8 and 7 days after the first spray in each respective year.  The second trial in each year was designed to examine the number and timing of applications of endosulfan (Thiodan 3E) at 1.12 kg (AI)/ha or esfenvalerate (Asana XL) at 56 g (AI)/ha for C. assimilis control.  Treatments were an untreated control and each insecticide applied at early bloom (50-70% of plants with flowers), late bloom (90-75% of plants with flowers remaining), and both applications times.  Endosulfan was examined in 1992 and 1993, and esfenvalerate was examined in 1993 and 1994. 

            Treatments in all trials were arranged in a randomized complete block design with four replications.  Insecticides in 1992 and 1993 were applied with a CO₂-powered backpack sprayer equipped with 003 flat-fan nozzles.  Spray pressure was 1.41 kg/cm² which delivered 253 L/ha.  In 1994, insecticides were applied with a tractor-mounted, CO₂-powered sprayer which delivered 201 L/ha.

            Adults were sampled with a 38 cm diam. sweep net by taking 6-8 sweeps per plot.  Plots were sampled before the first application, 2-3 days after the first application, before the second applications, 2-3 days after the second application and 7-9 days after the second application in each year.   The percentage of pods infested with larvae was measured by inspecting 200 pods in each plot. The center two drill passes (27.9 m²) of each plot were harvested using a small-plot combine. Grain weight, moisture content, and 1000 kernel weight were measured.  Oil content of grain also was measured (Buntin 1999).  Grain yield and oil content were adjusted to 8.5% moisture content.

            Data were analyzed by study and sample date with a analysis of variance for a randomized complete block design.  Means were separated using Tukey honestly significant difference multiple comparisons test.   A factorial treatment analysis was used in the application timing trial with main effects of application timing averaged among the two insecticides in 1993.

Yield loss assessment and economic injury levels  The relationship between C. assimilis pod infestation and yield loss were examined by regressing the yield difference (percentage or actual amount) between the untreated check and the pyrethroid treatment with the greatest yield against the percentage of  infested pods of the untreated check in each of the six insecticide trails.  In addition to results from these six trials, data from experiments in four years (1994 - 1997) examining the feasibility of a trap crop system for managing C. assimilis (Buntin 1998) were also included in the regressions.

            Economic injury levels (EIL) were calculated as EIL = C/VIDK, where C = control costs (US$/ha), V = commodity value, ID = yield loss per unit of insect density, and K = proportionate control (Pedigo et al. 1986).  C for material and application costs were $14.82/ha and $24.70/ha for one application of esfenvalerate or endosulfan, respectively.  ID was estimated from the regression of actual yield loss and pod infestation.  K was assumed to be 100%.

RESULTS

Insecticide efficacy trials  The pyrethroid insecticides (permethrin, esfenvalerate, bifenthrin, and zetacypermethrin) were not significantly different in efficacy in any trial and were the most effective insecticides in reducing C. assimilis infestations.  Organophosphate insecticides (ethyl parathion,  methyl parathion, methomyl, and phosmet) and the organochlorine insecticide endosulfan were variable in efficacy and usually less effective than the pyrethroid insecticides in controlling C. assimilis. Azadirachtin and carbaryl were not effective at reducing C. assimilis infestations. Differences in grain yield between pyrethroid-treated and untreated plants averaged 12.2, 2.8, and 14.8% for the three respective years. Grain oil content and kernel weight were not significantly different than the untreated controls in any year. Additional details are presented by Buntin (1999).

Application timing trials   In both years, two applications of esfenvalerate were needed to reduced  larval pod infestations to low levels and maximized net returns over untreated (Table 1).

Table 1.  Net returns ($) over untreated of one or two applications of two

insecticides for C. assimilis control in canola.

                                 Treatment                  Net return ($) over untreated             

Insecticide                    times               1992                    1993                1994      

Endosulfan                     Early                 60.05                   135.05                 –

                                      Late                    1.80                    -29.45                 –

                                      Both                 36.10                     71.10                 –

Esfenvalerate                  Early                   –                          76.43                 8.68

                                      Late                    –                          46.18               39.18

                                      Both                   –                        141.61             135.36

A second application of endosulfan did not significantly improve control as compared with one application, and no treatment of endosulfan was as effective as the two-application treatment of esfenvalerate.  Furthermore, the two-application treatment of esfenvalerate yielded 685 and 660 kg/ha more than the untreated control in 1993 and 1994.  Kernel weight was not significantly affected by treatments in any year. Grain oil content was not significantly affected by treatments in the 1992 and 1993, but the early treatment of esfenvalerate enhanced oil content by 2% in 1994. Additional details are presented by Buntin (1999). 

Yield loss assessment and economic injury levels  Yield loss expressed as a percentage increased in a linear relationship with percentage of infested pods when pod infestations exceeded 22.2% as indicated by the X intercept. The equation best describing this relationship was Y = -12.178 + 0.548X, (R² = 0.56; F = 12.63; df = 1,8; P = 0.0075), where Y is percentage yield loss and X is percentage infested pods.  Slope (b) of this equation was not significantly (t = 0.66, df = 7, P = 0.53) different than the slope (b = 0.65) of the same relationship calculated by Lerin (1984).  

            The actual amount of yield loss also increased in a linear relationship with percentage of infested pods and was best described by the model of Y = -326.94 + 14.35X, (R² = 0.66; F = 18.61; df = 1,8; P = 0.0026), where Y is yield loss (kg/ha) and X is percentage of infested pods, when the percentage of infested pods exceeded 22.8% as indicated by the X intercept (data not shown).  Using this regression, economic injury levels were calculated for one and two applications of an insecticides and various commodity values (Table 2).

Table 2.  Modified economic injury level for assessment of C. assimilis larval

damage of canola.

                    Modified EIL (% infested pods)                      

                          Control costs (US$/ha)                             

 Commodity                  $14.82              $29.64              $24.70              $49.40

 value ($/kg)                (1 spray)          (2 sprays)         (1 spray)           (2 sprays)     

0.20                                  27.9                   33.1                   31.4                   40.0

0.25                                  26.9                   31.0                   29.7                   36.6

0.30                                  26.2                   29.7                   28.5                   34.3

0.35                                  25.7                   28.7                   27.7                   32.6

CONCLUSIONS

            These results indicate C. assimilis injury primarily affected canola grain yield with little effect on grain oil content and kernel weight.  Yield loss increased linearly when infestation exceeded 22-23% infested pods, which supports the findings of Lerin (1984) and Free et al. (1978) that canola can tolerate pod infestation of <26% infested pods without measurable yield loss. Clearly, a canola stand can compensate for a moderate level of pod injury and seed loss by C. assimilis larvae. Although not measured directly, because larval injury did not affect kernel weight, plants must have compensated by either producing more pods and thus seeds per plant and/or by retaining and filling normally aborted seed.   When pod infestations exceed 26%, yield loss increased linearly by about 1.7% or 30 kg/ha for each 1% increase on percentage of infested pods.  European studies report a decision rule of about 1.0 adult per plant for C. assimilis management in the absence of further damage by D. brassicae in oilseed rape (Free and Williams 1978, Lerin 1995).  Further development of decision rules by relating C. assimilis pod infestation levels to adult numbers  is needed to prevent unnecessary insecticide use on canola in North America.

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