Reproductive potential, sex ratio, and mating efficiency of the straw itch mite (Acari: Pyemotidae)

Pyemotid mites are common parasites of many species of Lepidoptera, Coleoptera, and Hymenoptera. They have been associated incidentally with virtually every insect order, paralyzing and ultimately killing their hosts. The straw itch mite, Pyemotes tritici (Lagreze-Fossat and Montane 1851), has been studied the most in this family. The feasibility of using P. tritici females in mass-rearing projects related to biological control programs (e.g., stored-product insects) depends on fundamental knowledge of their reproductive and population biology. P. tritici reproduces by arrhenotokous parthenogenesis and sex ratio is extremely female biased. Females become physogastric as they feed on host hemolymph. Offspring development is ovoviviparous: diploid adult females and haploid adult males emerge from a birth canal on the opisthosoma. Two studies were designed to assess female and male reproductive potential (progeny number and sex ratio), emergence rates, and male mating agility. The first study measured daily and total progeny number and sex ratio from 128 isolated gravid females. Results indicated that females produced up to 355 offspring, averaging 254 total progeny of which only 8.7% were male. Males emerged faster than females and >50% of all progeny emerged in the first 6 days. The mean duration of progeny emergence was 15.3 d. In the second study, mating ability of 25 males was evaluated by supplying each male with a continuous series of virgin females until the males died. This study revealed that males can fully inseminate >14 consecutive virgin females. Thus, the small proportion of males is capable, at least theoretically, of inseminating all females in a sibship. Mating per se appeared to potentiate fecundity; total offspring per female decreased significantly with mating order. Furthermore, mating order affected sex ratio: after the 14th consecutive mating, the proportion of males in progenies increased steadily, reflecting a decline in male reproductive ability. Males that were allowed to recuperate overnight exhibited a temporary recovery to nearly normal insemination levels before rapidly declining in mating ability. The reproductive potential of P. tritici is extraordinarily high. The intrinsic rate of increase was estimated to be 0.63. Thus, P. tritici has a much higher innate capacity for reproduction than any host for which it might be employed as a biological control agent.