ONION FLY, DELIA-ANTIQUA, OVIPOSITION AND MATING AS INFLUENCED BY INSECT AGE AND DOSAGE OF MALE REPRODUCTIVE-TRACT EXTRACT (DIPTERA, ANTHOMYIIDAE)

One hundred percent of virgin female onion flies, Delia antiqua, recei ving greater than or equal to 1/20 of a male equivalent of an aqueous extract of mature male reproductive tract remained unmated in the pres ence of males and began laying unfertilized eggs at a normally mated r ate of about 20 eggs/female/day. The 50% behavioral response (BR(50)) fell between 1/40 and 1/20 of a male equivalent. Sex peptide responses are not always all-or-none. Some females receiving extract at less th an or equal to 1/40 male equivalent oviposited at an intermediate rate , Moreover, at low sex peptide dosages, some females were fully activa ted ovipositionally but were receptive to mating. A low level of sex p eptide was present in I-clay-old males. Sex peptide titer rose with ag e until plateauing by 6 days posteclosion. Males began mating at 3 day s, when they first had ample mature sperm; 50% of 6-day-old males mate d. The mean number of females inseminated per male exposed to an exces s of virgin females over 24 h was 4.3 +/- 0.6 (+/- SE), Presence of ma ture eggs was not always a prerequisite for mating, although probabili ty, of insemination was correlated with egg maturation. One-day-old pr eovipositional females receiving 1/20 of a male equivalent of extract began ovipositing when they had mature eggs at 5-6 days old. Therefore , sex peptide may act early and permanently or have a long half-life a nd affect behaviors once females reach sexual maturity. Male flies pro vide females with an excess of sex peptide in many cases. D. antiqua m ales transferred ca. 5-10 times more sex peptide than necessary to act ivate females fully. We suggest this excess is related to the speed of female response. It is yet unclear whether sex peptide potency or tit er in Diptera has become exaggerated by intra- ol intersexual selectio n.