The Evolution of Oviposition Tactics in the Bean Weevil, Callosobruchus maculatus (F.)
1975
Mitchell, Rodger
A sizeable fraction (7%—17%) of the mung beans that are the preferred food of Callosobruchus maculatus are not large enough to support two larvae. Whenever two or more eggs are on a bean, development takes 40 days rather than 33 days, and only 8% of the second eggs develop even in beans with resources enough for two larvae. This larval competition confers an advantage on females that disperse eggs uniformly. Females tend to avoid adding second eggs as long as there are unusued beans. Survivorship of single eggs on beans increase with weight (l₁ = .—005 mg + .368) and females oviposit on the largest of the unused beans first. Survivorship of the second egg (l₂ = .31), due principally to mortality of the first egg, is about half that of the first egg; a second or third egg is added after nearly all the beans carry one or two eggs respectively. With these constraints the beetle may maximize fitness if it knows the weights and egg loads of all available beans. The actual performance falls short of achieving maximum fitness and the oviposition of the beetle fits a computer simulation that makes decisions after comparing the weight and egg load of the present bean with the last bean encountered. The performance of the beetle can be appropriately evaluated against models of unspecialized beetles that ovipost randomly and for beetles with maximum fitness. This shows that the beetle gains about 70% of the fitness that it could evolve.
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