The influence of cucumber offered as a host plant either alone or with Aphis gossypii Glover (Hemiptera: Aphididae) was studied on the various life table and biological characteristics of the predatory bug Macrolophus pygmaeus Rambur (Hemiptera: Miridae). The nymphal development was studied at 15, 20, 25, 30 and 35°C while adult performance was assessed at 15, 20, 25 and 30°C, using a 16L : 8D photoperiod and 65 ± 5% r.h. Nymphs completed their development at all temperatures except at 35°C. Nymphal development took significantly longer time in the absence than in the presence of prey at 20 and 25°C, but the reverse was true at 15°C. Nymphal mortality was highest at 15°C in the presence of prey and it was mainly recorded at the first and second stages. Females oviposited a small number of eggs at all temperatures but not at 30°C in the absence of prey. The average number of eggs per female was almost similar with or without prey, being highest at 20°C, and adult longevity was highest at 15°C. The results concerning population parameters clearly showed that cucumber with or without prey can not support a population increase of M. pygmaeus. However, it seems that A. gossypii on cucumber inhibits development of M. pygmaeus more than when this aphid species is not present. This adverse effect on this host plant-prey system possibly results from the particular aphid genotype on cucumber, leading to high nymphal mortality, reduced fecundity and short adult life-span of M. pygmaeus.
This study determined the effect of larval density-dependent competition for food on development and adult fitness in Sesamia nonagriodes Lef. (Lepidoptera: Noctuidae). Different numbers (5, 10, 15, 20 and 30 individuals) of larvae of the pink stalk borer were reared on a constant amount of food. Although crowding during the immature stages did not significantly increase mortality, it prolonged the larval developmental period and resulted in reduced pupal weight. Females were more adversely affected by high density than males, resulting in lighter females, indicating that female growth is more sensitive to density. The fecundity of the adults reared in the various larval crowding treatments was analysed. Total female fecundity was correlated negatively with increasing larval density. The effects of crowding on fecundity were not caused by the reduced pupal weight, indicating that food shortage during larval development may affect adult traits. Female longevity was negatively affected by density and positively related to pupal weight. Thus, larval density may affect the allocation of food resources and adult fitness. We conclude that crowding related changes during larval development directly affect larval life and reduce female fitness.
The predation rate of the polyphagous predator Macrolophus pygmaeus (Rambur) when offered two aphid species, Myzus persicae (Sulzer) and Macrosiphum euphorbiae (Thomas), was investigated. Equal or unequal numbers of nymphs of each instar of the aphids were offered to the predator alone or together. Aphids were placed on an eggplant leaf, together with a fifth instar nymph of the predator in a plastic Petri dish and kept in growth cabinets at 25°C, 65 ± 5% r.h., and a 16L : 8D photoperiod. The predation rate of M. pygmaeus was always higher on M. persicae than on M. euphorbiae. However, biomass consumption was highest when instars of M. euphorbiae were offered in unequal numbers. The predator showed a strong preference and higher biomass consumption of first and second instar M. persicae. In tests where M. euphorbiae was the prey, preference and biomass consumption were almost always higher for the first instar. Therefore, first and second instar M. persicae and first instar M. euphorbiae provide optimal prey for M. pygmaeus. The implication of the prey preference shown by M. pygmaeus for the biological control of these two aphid species is discussed.