Environmental regulation of the larval diapause of the southwestern corn borer, Diatraea grandiosella Dyar (Lepidoptera : pyralidae)
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Diapause is a state of arrested growth or reproduction which is commonly found in insects (Lees, 1955). It may occur at any stage in the life cycle of an insect and is usually an important adaptation in the development of an insect. Diapause is of special value to insect species as it provides rhythmic synchronization of the life cycle with that of favorable environmental conditions. Diapause is characterized by a significant reduction in respiratory rate, decrease in the water content, presence of abundant deposits of reserve materials, absence of feeding, reduced locomotary activity, and arrest of gonodal development. All these changes lead to a low rate of metabolism (Danilevskii, 1961; Beck, 1968). Although diapause has been extensively studied most data have been obtained for species which evolved in temperate regions (Andrewartha, 1952; Lees, 1955; Danilevskii, 1961; Beck, 1968; Mansingh, 1971)- Such species are subjected to marked seasonal changes in photoperiod and temperature. Experiments usually show that photoperiod is the principal factor regulating their diapause induction while temperature influences the range of inductive photoperiods. The relationship between climate and diapause of tropical insects has not been as thoroughly studied (Danilevskii, 1961). Relatively minor seasonal changes in day length occur in tropical and subtropical latitudes. While these changes in day length may serve to program the development of some species (Norris, 1959), other climatic factors such as temperature and rainfall may play a more dominant role in regulating the onset and termination of their diapause (van der Laan, 1959). The present investigation was undertaken to obtain data about environmental factors which regulate diapause induction and development of the southwestern corn borer, Diatraea grandiosella Dyar, a neotropical species. Pyralid moths of the genus Diatraea are found mainly in tropical and subtropical latitudes. D. grandiosella is one of about six Diatraea species which have migrated into the United States (Dyar and Heinrich, 1927; Holston, 1955). This insect is currently found in Mexico and the United States (Elias, 1970). While its distribution in Mexico remains static, it is gradually spreading in an easterly direction in the United States. Since entering the United States from Mexico around 1JL3; the borer has become established in 13 southern states (Henderson et al., 1966). In all these states it has become a major pest of corn and sorghum. Besides displaying all the typical physiological features of diapause, overwintering southwestern corn borers assume a characteristic larval form. Larvae of the summer generation are white with brown head capsules and prothoracic shields and have segmentally-arranged sclerotized pinaculae. Diapause larvae lose their characteristic pigmentation and become immaculate. The insect is bivoltine in Missouri (Jacob, 1971). It enters into its facultative diapause in late summer and pupates the following spring. Most of the diapause larvae girdle the stalks near the ground level as a part of their preparation for diapause. Adult moths from the overwintering generation emerge in mid-May. The aim of this study was (1) to examine the occurrence of the different morphological forms of mature southwestern corn borers, (2) to investigate the influence of photoperiod and temperature on the occurrence of these forms, (3) to determine the role of photoperiod and temperature in regulating diapause induction, diapause determination and diapause development, (4) to assess the effect of contact water on diapause development and the resumption of morphogenesis, and (5) to study the effect of low winter temperatures on the survival and distribution of the species within the United States.
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