Temperature-Development Relations of Eutrombicula belkini (Acarina: Trombiculidae) and Their Possible Ecological Significance

Abstract

Eutrombicula belkini, a summer pest chigger from the Sacramento Valley, metamorphoses from engorged larva to the deutonymph at constant temperatures from 20 to 37.5 C; the protonymph stage occupies 22.09 ? 0.14, 12.59 ? 0.12, 7.91 ? 0.08, 5.92 + 0.08, and 5.04 + 0.09 days at 20, 25, 30, 35, and 37.5 C, respectively. For the tritonymph at the same temperatures these data are 25.31 ? 0.20, 14.44 + 0.28, 9.24 ? 0.12, 6.59 ? 0.18, and 6.82 ? 0.18 days. The protonymph survival is 0, 91, 100, 79, 65, 73, and 0%o at 15, 20, 25, 30, 35, 37.5, and 40 C, respectively, and survival of the trito- nymph is 94, 92, 95, 83, 65% at 20, 25, 30, 35, and 37.5 C, respectively. These times are longer, especi- ally in the protonymph, than developmental times reported for some winter species. Also, E. belkini shows no survival at 15 or 10 C, temperatures at which some winter species will survive and metamor- phose to the deutonymph. Trombiculid mites (chiggers) characteristi- cally occur as larvae in definite, predictable times of the year: some species appear in the cool or cold season and others emerge in the warm months. In some regions winter is the season of greatest precipitation and summer is dry; and trombiculid mites that occur in winter or in summer may be controlled by humidity in addition to, or instead of, tempera- ture. The phenomenon of seasonality of larval trombiculid mites is abundantly documented. Some Oriental chiggers are associated with scrub typhus and, for this reason, the biology of Far Eastern species has long been studied. Sasa (1961) reviewed the Japanese literature. In an early classic study, Elton and Keay (1936) described in detail the seasonal occurrence of the European harvest mite, Neotrombicula autumnalis; it is essentially a summer species. Jameson and Brennan (1957) recorded the monthly occurrence of a number of species in California, and the great majority were found to occur (as larvae) in the cold, wet winter months. Most pertinent to this investigation is the study of Jenkins (1948) of the biology of Eutrombicula in North America. The cause of this seasonal dichotomy is a major enigma in chigger biology. Three obvious exogenous factors are temperature, humidity, and photo- period. In this study, Eutrombicula belkini (a summer species) was reared at a series of con-