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A New Approach for Detecting Insecticide Resistance in Onion Maggot Populations 2003

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Project Leaders: B. A. Nault, Dept. of Entomology, NYSAES-Geneva

J. P. Nyrop, Dept. of Entomology, NYSAES-Geneva

A. M. Shelton, Dept. of Entomology, NYSAES-Geneva

R. W. Straub, Dept. of Entomology, NYSAES-Highland

Cooperators: J. Z. Zhao, Dept. of Entomology, NYSAES-Geneva

J. van der Heide, NYS Cooperative Extension, Oswego County

C. MacNeil, NYS Cooperative Extension, Ontario County

C. Hoepting, NYS Cooperative Extension, Lake Plains Veg. Prog.

M. Ullrich, NYS Cooperative Extension, Orange County

Type of Grant: Monitoring, forecasting, and economic thresholds

Project Locations: Research was conducted in most of the major onion growing regions in NY. The survey information is applicable only to NY onion growers, but the technique developed is applicable worldwide.


The primary goal of this research was to develop a rapid assay for identifying insecticide resistance in onion maggot populations and then to use it for predicting controllability of onion maggot infestations in the field. In 2002 and 2003, chlorpyrifos (Lorsbanยจ 4E) was evaluated for managing onion maggot infestations in a total of 23 commercial onion fields. Populations from 12 of these fields were established and LC50s for them were determined using the larval assay. The LC50 value for the chlorpyrifos-susceptible population (ST) was 14 ppm, whereas LC50 values for populations collected from commercial onion fields ranged from 63 ppm to 4343 ppm. These results indicate that resistance to chlorpyrifos occurs in some commercial onion fields and that the levels of resistance can vary dramatically. Control using chlorpyrifos in the field also differed considerably among fields. In both years, the percentage of onion plants killed by onion maggot in untreated and chlorpyrifos -treated plots were positively correlated, indicating that chlorpyrifos lost effectiveness as onion maggot pressure increased. To determine if the lack of onion maggot control in chlorpyrifos-treated plots was primarily due to resistance, a negative correlation should occur between an increase in the resistance ratios and the percentage reduction of onion plants killed by onion maggot using chlorpyrifos in the field. However, this was not the case. Therefore, based on the absence of this negative correlation, the larval assay will not be a reliable tool for predicting the controllability of an onion maggot infestation using chlorpyrifos in the field. However, the larval assay is an effective tool for identifying levels of resistance. It is likely that the efficacy of chlorpyrifos for controlling onion maggot in the field is largely affected by factors in addition to resistance, such as environmental conditions and the level of pest pressure.

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