Multiple Natural Enemies Research Articles

The relationship between dietary specialism and availability of food and water on cannibalistic interactions among predatory mites in protected crops.

The interplay between dietary specialism, the tolerance of food and water stress and level of cannibalism is likely to be important in determining the outcome of biological control using inundative releases of multiple natural enemies, such as phytoseiid mites in protected crops. The dietary specialist, Phytoseiulus persimilis, with a short immature development time (4-5 days) when plentiful food was available had a low ability to survive without food (5 days), even with access to water. The dietary generalists, Neoseiulus californicus, N. cucumeris and lphiseius degenerans, had longer immature development times (by up to 2 days) than P. persimilis. Survival ability differed amongst the generalist species when they were starved but provided with constant access to water. Both N. californicus and N. cucumeris survived the longest (8-10 days) and I. degenerans survived the shortest period (4 days). No negative intra-specific interaction between immatures was observed with P. persimilis when food was available and in the absence of food this species tended to starve rather than act cannibalistically. Both N. californicus and N. cucumeris showed a low degree of cannibalism between immatures, either when food was available, or when starved but given access to water. Even when food was available survival of I. degenerans fell by 30% in 4 days and remained at 60-70% for 3 further days; survival continued to decline rapidly when they were starved but provided with water. This indicates that immatures of I. degenerans could either feed on dead conspecifics or that they were capable of a degree of cannibalism. Adult females of P. persimilis did not feed on conspecific eggs even when deprived of food but provided with water. Adult female N. californicus and N. cucumeris did feed on conspecific eggs but at a low level (<1 egg per day), which occurred only after 48 h starvation. Although egg cannibalism occurred more consistently with adult female I. degenerans than with other mite species it was at a low level (<1 egg per day). If the tendency to cannibalism, not just of eggs but with more susceptible life stages such as larvae, is reduced when water is available freely this could be important in determining the interactions that occur under natural conditions.

Non-additive effects of multiple natural enemies on aphid populations.

The question of whether multiple natural enemies often interact to produce lower host mortality than single enemies acting alone has not yet been resolved. We compared the effects of four different combinations of natural enemies-parasitoids, predators, parasitoids plus predators, and no enemies-on caged aphid populations on marsh elder, Iva frutescens, in west-central Florida. Using starting densities of natural enemies commonly found in the field, we showed that parasitoid wasps reduced aphid population densities more than predatory ladybird beetles. The addition of predators to cages containing parasites reduced the ability of parasitoids to decrease aphid population densities. Because the experiments ran only over the course of one generation, such a reduction in the effectiveness of parasites is likely caused by interference of predators with parasitoid behavior. Parasitism in the cages containing both parasitoids and predators was reduced when compared to percent parasitism in parasitoid-only cages, but this could also be due to predation. Our experiments showed that ladybird beetles prey on parasitized aphids. Thus over the long-term, the effectiveness of parasites is impaired by the interference of predators on ovipositing parasitoids and by the predation of parasitized aphids. The effects of natural enemies in this system are clearly non-additive.

Bioecological and population studies of the cottony-cushion scale,Icerya purchasiMask., and its natural enemies,Rodolia cardinalisMul. andCryptochaetum iceryaeWill., in southern California

Competitive interactions between the two natural enemies of Icerya purchasi—Rodolia cardinalis and Cryptochaetum iceryae—were studied in detail in order to throw more light on the question of the introduction of multiple natural enemies as a biological control policy. The host, a major citrus pest accidentally introduced in 1868 from Australia, has been under control since 1888-1889, when its highly specific natural enemies (also from Australia) were imported for that purpose. Three different climatic regions of southern California were the study sites: the desert, the coast, and the interior. Life table studies of the scale indicated that it is maintained under control at very low population levels in the desert by vedalia and on the coast by Cryptochaetum, with competition causing displacement in both areas. In the interior, the two enemies seasonally share their prey in different proportions and are fairly even in their competitive abilities—vedalia usually taking more prey during the summer and fall, the fly taking more during the winter and early spring. In the interior, three generations of the host occurred in a year, while those of vedalia and Cryptochaetum averaged 12 and 8, respectively. Some host plants restricted vedalia development, and certain species of ants interfered with Cryptochaetum. Competition results in micro-habitats also varied, depending on prevailing environmental conditions and the time at which an enemy finds a scale colony. The “coexistence” of vedalia and Cryptochaetum in the interior is thought to be only apparent and maintained by the dispersal of Cryptochaetum from their coastal area of dominance. Coexistence experiments in the interior showed displacement of the fly by vedalia to occur after nine generations. Competition between the two natural enemies did not increase host survival. These extensive studies provided strong support for the policy of importation of multiple natural enemies.