Psyllids in Natural Habitats as Alternative Resources for Key Natural Enemies of the Pear Psyllids (Hemiptera: Psylloidea).

Simple Summary: The pear psyllids comprise over 20 species of small, sap-feeding insects which include several of the most damaging pests in commercial pear orchards of Europe, temperate Asia, and the Americas. Efforts to control these pests historically have centered on the use of insecticides. Because psyllid outbreaks often may be prompted by insecticidal destruction of natural enemies, pear growers have begun to substitute biological control and conservation of natural enemies for some insecticide applications. Conservation of natural habitats to make alternative resources (prey and hosts) available to natural enemies of crop pests increasingly is being examined as a tactic for strengthening biological control in crops. This review shows that psyllids in natural habitats are important resources for species of predators and parasites which provide biocontrol of pear psyllids. Traits which affect suitability of non-pest psyllids to beneficials include how closely related they are to pear psyllids and type of seasonal life cycle, the latter shown to govern when (seasonally) non-pest psyllids in natural habitats are available to beneficials. Confirmation that natural habitats and these alternative resources lead to numerical increases in beneficials in orchards requires proof that key predators and parasites colonize orchards from natural habitats. Difficulties in obtaining this proof are discussed. The pear psyllids (Cacopsylla spp.; Psylloidea) comprise ~24 species of sap-feeding insects distributed in Europe, temperate Asia, and (as introductions) in the Americas. These pear-specialized insects are among the most damaging and difficult to control pests in orchards. Biological control increasingly is being used to replace or partially replace insecticidal management of pear psyllids. Many key natural enemies of pear psyllids regularly occur in non-orchard habitats on native plants. The presence of beneficial species both in orchard and non-orchard habitats (here referred to as "spillover") has prompted suggestions that native plants and their associated psyllids should be conserved as alternative resources for natural enemies of pear psyllids. The expectation is that the natural enemies will move from those habitats into psyllid-infested orchards. This review shows that psyllids in native habitats are important resources for several key predators and parasitoids of pear psyllids. These resources are critical enough that some beneficials exhibit an almost nomadic existence as they move between plant species, tracking the seasonal appearance and disappearance of psyllid species. In contrast, other natural enemies show minimal or no spillover between orchard and non-orchard habitats, which likely is evidence that they exhibit limited movement at best between orchard and non-orchard habitats. To show conclusively that spillover also indicates that a beneficial species disperses between native habitats and orchards requires difficult research on insect movement. This review concludes with a brief discussion of these difficulties and possible solutions. [ABSTRACT FROM AUTHOR]