Use of Isotopes for Investigating the Behaviour and Ecology of Insect Pests in Some Recent Studies

Investigations into the ecology, behaviour, dispersal and longevity of insects have always provided challenges to the entomologist. The use of isotopes is an effective tool and the following is a report on some interesting problems solved by their application. Anopheles stephensi is the main vector o f malaria in southern Iran. On P32-labelled mosquitoes, the dispersal, behaviour, digestion of blood meal, maturation of ovaries and length of gonotrophic cycles were successfully worked out. It was found that in about 80% of the cases the mosquito needed two blood meals for the completion of the first cycle. The first cycle itself was completed in 4-5 d depending upon the temperature. Labelled mosquitoes which had emerged overnight were released in an isolated village. The ratio of active mosquitoes to total catch was worked out every day and thus, on the assumption that the natural population remained constant, the mortality rate, which was found to be exponential for the first six days, was worked out. The mating behaviour of the female was also studied by using normal females which had mated once with P32-labelled males. It was found that the female mates more than once and that after mating with an active male, the spermathead became active. Counts of up to twice the background (12 counts/min) were obtained by using males giving about 15 000 counts/min. Studies of the injection of saliva in glucose solution during feeding were also made on P32-labelled mosquitoes. Eurygaster integriceps is a serious pest of wheat in Iran, Pakistan and the Middle East. Using P32 activated wheat plants, the feeding behaviour of the first-instar nymph was investigated. Other foods, radioactively labelled, were also studied and it was found that feeding was essential for the first moult, even if the food consisted of water absorbed in a filter paper. (author)