OVIPOSITIONAL PATTERNS AND LARVAL MOVEMENT OF DELIA-ANTIQUA (DIPTERA,ANTHOMYIIDAE) ON SPROUTED BULB AND SEEDLING ONIONS

Delia antiqua (Meigen) exhibited pronounced differential oviposition o n seedling versus sprouted bulb onions. In a dual treatment experiment , flies laid significantly more eggs per plant per day on sprouted bul bs (97.4 +/- 14.4 [mean +/- SEM]) than on seedlings (0.4 +/- 0.3). In a single-treatment experiment, sprouted bulbs received 103.2 +/- 11.4 eggs per plant/d and seedlings only 3.5 +/- 1.8. Switching treatments in the single-treatment experiment caused an immediate reversal in ovi position. Flies first given sprouted bulbs ceased oviposition when pre sented with seedlings and flies switched from seedlings to sprouted bu lbs commenced oviposition. Therefore, once activated, the fixed action pattern governing D. antiqua egg deposition is not free-running. In g lasshouse experiments, sprouted bulbs did not influence the pattern of egg deposition by female flies on surrounding seedlings, indicating t hat ovipositional patterns cannot fully explain aggregated damage obse rved in the field. Onion maggot movement away from sprouted bulbs prev iously inoculated with high numbers of eggs resulted in aggregated dam age to surrounding seedlings, at least partially explaining field obse rvations. Differential oviposition supports the use of sprouted bulbs as an ovipositional trap crop for onion maggot control, but larval mov ement away from potentially impenetrable, undamaged bulbs requires con sideration of trap crop placement relative to seedlings, timing of pla nting so that the seedling stage coincides with peak D. antiqua ovipos ition, and timing of trap crop removal.