Rotation, distance, and insecticide resistance in Colorado potato beetle

Rotation, distance, and insecticide resistance in Colorado potato beetle

Summary

Resistance to imidacloprid is widespread, though it still provides significant protection. One of the cornerstones of soil, disease, and insect management is the rotation of crops. Crop rotation decreases the size of colonizing populations of many insects, including Colorado potato beetle, Leptinotarsa decemlineata (Say). However, rotation may also reduce the level of insecticide resistance if resistant individuals are unable or less likely to migrate long distances to colonize new crops. Resistance to imidacloprid has many costs in CPB, including reduced fecundity, fertility, and walking speed. We tested the prediction that rotation reduces the resistance of colonizing CPB by collecting emerging walking adults in the spring via drift fences, flying adults using harp traps, and colonizing adults using untreated trap plants at increasing distances from the overwintering site at distances of up to 500 meters. We also tested the resistance of colonizing adults on rested and un-rested fields. We found that the offspring of flying recently emerged adults were in fact less resistant than walking emergers, and that adults collected from rotated fields were slightly less resistant than those from rotated fields, but we did not find an effect of colonization distance on resistance. We experienced some difficulties with the filter paper assay used, and will replicate the trials with more fields and adult and second instar assays.

Objectives/Performance Targets

Objective I: Set up drift fences with pitfall traps to collect walking emergers, and harp traps to collect flying emergers, to see if flying CPB in the spring are less resistant than walking emergers.

Objective II: Collect Colonizing CPB 100, 200, and 400 meters from their overwintering sites will be compared in resistance to see if colonizing distance is negatively correlated with resistance.

Objective III: Measure Resistance of CPB colonizing fields rested from, or planted in, potato the previous year will be measured, and correlated with the distance to the closest field the previous season.

Accomplishments/Milestones

Objective I: In 2007 we set up drift fences around a small woodlot surrounded by about 325 acres of potato field in Riverhead NY. The woodlot attracts diapausing adults each fall. We collected early and late walking and flying beetles using drift fences made of aluminum roof flashing and pitfall traps along with harp traps to catch flying beetles. Separate colonies were established with walking or flying adults captured at emergence from the diapause site. Clutches were collected and both 1st instar filter paper assays, where hatchlings were allowed to feed for six hours and then placed on filter paper which had from 1.6 to 49.5 µg/cm^2 imidacloprid applied in 0.1 ml acetone which was evaporated off prior to introducing the larvae. For adult assay each adult was treated with a single drop of 2 µl of imidacloprid dissolved in acetone at a concentration of 1E-3. Adults were held in 60 mm petris with fresh potato foliage replaced daily and were scored at five and seven days following application. Assays were carried out on second-instar larvae by subjecting them to a 1 µl drop of a range of concentrations of imidacloprid dissolved in acetone from 1E-6 g/ml to 1E-4 g/ml plus control. Mortality was assayed after 24 hours.

Objective II: Adults were collected from trap plants at distances 50, 100, 200, 400, 500, and 640 meters from the overwintering site, established in small butterfly cages with potted potato plants, and clutches were collected for 1st instar assays. 10-60 adults were collected from each location, and 2188 hatchlings were assayed.

Objective III: 10-20 potted potato plants were placed in freshly planted potato fields in and near Riverhead, NY and Scuttlehole, NY. A late spring emergence in 2007 delayed appearance of potato beetles until after most field plants had erupted, so adults were collected from field plants, likely after consuming some treated foliage. Adults were housed in butterfly cages as above, and 5791 1st instar larvae were assayed. After collecting clutches for assay, the surviving adults themselves were assayed using a single drop of 2 µl of imidacloprid dissolved in acetone at a concentration of 1E-3. Mortality was assessed after seven days.

Results

Disappointingly, the first-instar assay was not reliable this summer. Intermediate level doses had the highest mortality, suggesting that initial exposure at the highest levels limited uptake of toxin. There were no significant differences in resistance in first instar larvae among distances from the overwintering sites, nor among rotated and non-rotated fields, however, the lack of fit to the probit model in the data make it difficult to conclude anything from those assays.

We did observe some significant results in the adult and second-instar assays. Adults from non-rotated fields were slightly more likely to survive seven days following imidacloprid application (N=328, Chi^2=7.05, P=0.03). Second-instar larval offspring of walking emigrants from the overwintering site were 4.7 times as resistant as flying emigrants (using the ratio of the maximum likelihood fitted LC50’s).
From the first season there is the suggestion that more resistant beetles are not able to travel as far to colonize fields in the spring, and therefore that crop rotations can contribute to reducing need for insecticidal sprays. However, the avoidable difficulties with assays prevent drawing any strong conclusions from the first year

Impacts and Contributions/Outcomes

Right now the results are too equivocal to make recommendations for practice or include in outreach, so the primary impact is to cause us to replicate the research for one more year with a wider range of rotated and non-rotated fields, and with only more reliable assay techniques.

One factor hampering our ability to see effects of rotation in our study may be lack of independence among fields. Though we sampled from eleven fields, those fields were planted by only two farmes, and some were adjacent or across the road from each other. Other planting practices besides rotation or continuous planting can influence resistance, and so a more diverse group of fields will make the study more valid. We will expand the number of growers we work with in the second year, and made initial contacts from meeting with growers during collecting trips. Two additional growers have expressed a willingness to let us sample.

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