Larval Diapause of Maternal Origin: Induction of Diapause in Nasonia Vitripennis (walk.) (hymenoptera: Pteromalidae)

Factors which induce diapause in insects are, in most cases, only effective when applied to the individuals which are destined to cease development (Lees, 1955), although in many examples—for instance the vine leafroller, Polychrosis botrana (Komarova, 1949)—the sensitive stage and the resulting diapause are separated by several intervening instars. However, there are a few insects known in which factors affecting the maternal generation induce diapause in the progeny. Examples of this type of induction are found in Spalangia drosophilae and Cryptus inornatus (Simmonds, 1946, 1948) in which the age of the female parent and the temperature and dietary conditions she experiences before oviposition affect the incidence of diapause in the larvae she produces. There is also evidence of a maternal influence on production of diapause larvae in Lucilia sericata (Cragg & Cole, 1952), Phlebotomus papatasii (Roubaud, 1928) and Aedes triseriatus (Love & Whelchel, 1955). The most recent example was provided by Schneiderman & Horwitz (1958) working with the parasitic wasp, Nasonia ( = Mormoniella) vitripenms, which has a facultative diapause at the end of the fourth and final larval instar. Working with strains of Nasonia from North America (Massachusetts), Schneiderman & Horwitz tested many environmental and intrinsic factors affecting both larval and maternal generations and concluded that chilling the females whilst the eggs were developing in the ovaries, or depriving them, for a few days, of hosts in which to oviposit, increased the likelihood of the females producing diapause offspring. According to these authors neither daylength nor the age of the female parent was effective in inducing diapause and, although Wylie (1958) claimed that low temperature caused diapause in mature larvae, Schneiderman & Horwitz showed that none of the factors they tested were effective in the larvae themselves. In a strain of Nasonia isolated in London, however, it has been shown (Saunders, 1962) that diapause larvae are more frequent among the offspring of old females, and in the present experiments it was found that daylength affecting the maternal generation was undoubtedly the most important factor inducing diapause in this species. It became clear, therefore, that a re-assessment of the factors inducing diapause in Nasonia was necessary.