Psyllids are generally narrowly host specific and are restricted almost exclusively to perennial dicotyledenous plants. Psyllids are known vectors of plant diseases. In addition, the effects of salivary injection can be very severe, causing growth abnormalities in the plant. Psyllid development rates are related to the nutritive status of the host-plant and, as in aphids, the honeydew produced while feeding is low in nitrogen. Psyllids reproduce sexually and pass through five nymphal instars before becoming adult. The immature stages exhibit both morphological and behavioural adaptations to resist desiccation, and the life-cycle is often highly synchronised with host-plant phenology. Adult Psyllids are known to disperse up to eight miles on wind currents, although in general dispersal is less marked than in aphids. The mechanism of host-plant selection is probably chemo-gustatory. Capacity for population increase in Psyllids is more dependent on generation time than on female fecundity. Recorded Psyllid parasites include Diptera of the family Cecidomyiidae and a wide range of Hymenoptera. These parasite species are almost exclusively parasitic on Psyllids although there is little evidence for parasite–host specificity within the Psylloidea. Psyllid species from a wide range of geographical areas have similar predator complexes. Anthocorids, Syrphids, Coccinellids and Neuroptera larvae are the main predators. At high population densities nymphal mortality increases in a density-dependent manner with increasing density, whereas at low densities nymphal survival is enhanced by group feeding. Close parallels exist in the feeding biology of aphids and Psyllids, but aphids appear better adapted for the exploitation of unstable ephemeral food sources.
