The bio-ecology and mass-rearing of Chrysopidae, with special reference to Chrysopa zastrowi (Esb.-Pet.)

Abstract

Large numbers of green lacewings have been observed on cotton in South Africa. These insects are important predators of aphids and other soft-bodied insects, mites, and their eggs. It is therefore likely that they could potentially be of considerable value in suppressing some of the major pests on cotton. Since very little work has been done in South Africa on these predators, the studies reported here were undertaken. Chrysopa zastrowi (Esb. - Pet.) was arbitrarily chosen and is considered to be representative of the genus, and was the main species studied. Three methods of rearing these insects in the laboratory are described. In two methods each larva was confined to its own separate enclosure to minimise cannibalism, while in the third method large numbers of larvae were confined in a common enclosure. The larvae were fed mainly on potato tuber moth eggs but aphids, mealybugs and eggs of the Angoumois grain moth and of the housefly were also used. Since the adults are not predacious, they could be confined in the same cage for oviposition. They were fed on a diet of brewer's yeast, honey and water. At 25°C and 55% RH the egg hatches in four days; the larva passes through three instars in approximately ten to eleven days and requires on average about 488 aphids or 906 potato tuber moth eggs to mature. The mature larva spins a silken cocoon in which the third moult takes place after about four days. About ten days after spinning, the pupa leaves the cocoon by pushing off a lid. The pupal moult is accomplished outside the cocoon a short while after emergence. The adults mate and oviposit during the night only and lived in the laboratory for an average of 33 to 40 days (males and females respectively). Females showed a tendency to lay more eggs on a brown surface than on either yellow, white or green. Nipagin M (a fermentation suppressant) severely inhibited fecundity when added to the adult diet. The seasonal occurrence of adults of Chrysopa spp. was studied at Roodeplaat, near Pretoria, by two methods. Three main population peaks occurred during Dctober,December and March, the highest being in December. These peaks coincided with dry, sunny weather, suggesting an increase in activity due to the adults' search for moisture. There are probably three generations per year, although it is possible that they breed throughout the year since embryological development can proceed slowly at 15°C. The geographical distribution of various Chrysopa spp. was studied mainly in the Transvaal and South-western Cape. It is evident that certain species are probably very widespread. Toxicological studies indicated a fair tolerance of the egg stage to seven insecticides. Only monocrotophos and endosulfan could be tested on adults. Monocrotophos was found to be ten times more toxic to the adults than endosulfan. However, at the recommended field dosages on cotton neither of these two insecticides should cause undue mortality.