The role of temperature and larval crowding in morph determination in a tropical beetle, Callosobruchus subinnotatus.

Many insect species can produce individuals of more than one form or morph. Different morphs of the same species may differ in their physiology, morphology and in behaviour. Understanding the factors and mechanisms involved in determining the production of different morphs of insect species is of major importance in understanding the evolution of specific life-history strategies. In this paper, we studied the importance of temperature as an environmental factor involved in morph determination of the tropical beetle, Callosobruchus subinnotatus. Adults occur as relatively sedentary, highly fecund, 'normal' morphs or as an 'active' morph adapted to dispersal. Larval crowding, seed density and external temperature were independently manipulated in a series of controlled experiments and the proportions of 'active' and 'normal' adult morphs among the emergent adult populations were quantified and compared. Development in crowded conditions was found to be associated with the production of a significantly higher proportion of 'active' adults than development in isolation, and was also responsible for a predictable rise in the localised temperature of infested heaps of seeds of between 4 and 8 degrees C above ambient (27 degrees C). This rise in temperature is subsequently shown to be directly and quantitatively associated with the proportion of 'active' adults among emerging adults, both in the presence of larval crowding and independently from it. Thus, it is suggested that in the crowded environment representative of an infested seed store, it is rising temperature, occurring at a specific point in insect development which is the proximate cue for 'active' morph induction in C. subinnotatus. The results are compared to the strategies used by other polymorphic insects to survive in heterogeneous environments.

Variation in responses to susceptible and resistant cowpeas among West African populations of Callosobruchus maculatus (Coleoptera: Bruchidae).

The cowpea seed beetle, sometimes also known as the cowpea weevil, Callosobruchus maculatus (F.), is a major pest of stored cowpeas (Vigna unguiculata Walpers) in West Africa. Control methods have included development of 'resistant' varieties as an environmentally benign alternative to insecticides, but there is concern over their effectiveness because of population variation among the insects and the possibility of adaptation overcoming seed resistance. Populations of C. maculatus from Nigeria, Ghana, Benin, and Niger, were used to examine variation in response to resistant and susceptible cowpea varieties at two geographical scales. Among seven Nigerian populations, there were significant differences in development times, the pattern of adult emergence, adult weights, and female fecundity when reared under identical conditions. Development in the resistant variety was retarded, produced higher mortality and lower adult weights. Significant interactions between variety and population were evident in terms of their effects on adult weight and development time; development times in the resistant variety were longer and emergences occurred over a longer period in some populations than in others. Population responses to resistant seeds were therefore unpredictable, but there was no evidence to suggest adaptation to overcome seed resistance within three generations. On a larger geographical scale, variation in performance was much greater and therefore, even less predictable. Mortality in resistant seeds was also higher among populations collected from outside Nigeria and may be explained by significant adaptation among Nigerian populations to previous release of resistant varieties. The findings are discussed in relation to understanding the extent of intraspecific variation in C. maculatus and its implications for future pest management.

Bionomics and polymorphism in Callosobruchus subinnotatus (Coleoptera: Bruchidae).

Callosobruchus subinnotatus (Pic) is the major insect pest of stored bambara groundnuts, Vigna subterranea (L.) Verdcourt, in sub-Saharan West Africa, but little is currently known about its biology or how it may be controlled. A series of laboratory studies was performed to investigate the bionomics of and differences between two apparently different morphs of adult of each sex of this species, here termed 'active' and 'normal'. Major differences in their morphology, physiology and behaviour were identified and are described in detail for the first time. They provide clear evidence of the existence of an adult polymorphism among populations of this species, which is comparable in certain respects to that previously described for C. maculatus (Fabricius) and C. chinensis Linnaeus. Adults can be separated into the correct morph based on characteristic differences in elytral and pygidial colour and pattern. 'Normal' adults are characterized by having high fecundity, short adult life and are relatively sedentary while 'active' adults exhibit reproductive diapause (suspension of reproductive activity), are long lived, and show (at least in females) increased dispersal tendencies. These characteristics suggest adaptation of the 'active' and 'normal' morphs respectively to the different environments of field and seed stores, and the significance of the polymorphism in the life history of C. subinnotatus is discussed in this context. The design of any effective control regime for this bruchid needs to take account of and could potentially be based upon the existence of polymorphism in C. subinnotatus.