ONION MAGGOT DELIA ANTIQUA (DIPTERA: ANTHOMYIIDAE) ADULT ACTIVITY AND OVIPOSITION IN NEW YORK ONION FIELDS: IMPLICATIONS FOR MANAGEMENT
Insect pest activity is often concentrated in certain areas within crops. Past research has suggested that onion maggot, Delia antiqua (Meigen) (Diptera: Anthomyiidae), adult activity and oviposition may be concentrated along onion field edges and may be affected by the surrounding landscape. To examine this further yellow sticky cards were placed at varying distances along transects extending from edges of commercial onion fields that either bordered or did not border woods. Sticky cards were collected and replaced weekly throughout the 2002 and 2003 growing season. To determine if oviposition by first-generation onion maggot is concentrated along wooded field edges, potted onion plants were placed along edges and in centers of onion fields that either bordered or did not border woods. Results indicate that adult activity of both sexes is concentrated along edges of onion fields during the first- and second-generations of onion maggot, especially next to woods in the first generation. These ?edge effects? lessened as the season progressed and were not present during the third-generation. Oviposition was uniform with respect to distance from the edge and type of bordering habitat. It may be possible to limit application of insecticides targeting adult onion maggot to wooded field edges during the first-generation or to deploy cultural controls here. A delay in planting the onion crop has been recommended, but never evaluated, as a method of reducing onion maggot damage. A delay in planting will also result in plants being younger at the time onion maggot becomes damaging, and plants may be more susceptible to onion maggot when they are young. In contrast, planting early might result in onions being older and inherently more tolerant of onion maggot damage. To determine how planting date affects levels of onion maggot damage, damage was evaluated in three sequentially-planted onion plots in a commercial onion field in 2003. Earliest plantings coincided with the earliest commercial sowing date and delayed plantings were made three and six weeks later. To determine how plant age affects resistance to onion maggot, damage to onions of different ages was evaluated in the laboratory after they were infested with varying densities of onion maggot eggs. Ovipositional preference by onion maggot for differently aged onions was also investigated in the lab to determine if a preference for early plantings could result in egg densities being greater on these plants in the field. Onion maggot damage was high in earliest plantings (37%), lower but economically unacceptable in intermediate plantings (21%), and lowest in late plantings (2%). While the latest planting provided acceptable control, planting this late is not currently feasible for New York growers because they prefer to plant as early as possible to maximize size of onion bulbs at harvest. Onion maggot laid more eggs on older than younger plants in choice tests; therefore more eggs may be laid on early vs. delayed plantings in the field. Older plants were more resistant to maggot attack than younger plants at low egg densities, but not at high ones. This may explain why early plantings are more heavily damaged than delayed ones in the field; greater numbers of eggs may be laid on early plantings if they are preferred for oviposition, resulting in damage being higher in early than delayed plantings.
Brian Nault, Jan Nyrop, Alan Taylor
Department of Entomology New York State Agricultural Experiment Station
onion maggot; landscape ecology
dissertation or thesis