Scale Insects: Major Pests and Management

Abstract

Abstract Scale insects (Hemiptera: Sternorrhyncha: Coccoidea) are sap-sucking plant parasites that cause enormous agricultural and horticultural damage. The biological and ecological traits of scale insects combined with growing international trade of plants and plant products explain outbreaks of these insects observed on crops worldwide. Many major scale insect pests are mealybugs (Pseudococcidae), armored scales (Diaspididae) and soft scales (Coccidae); but there are also notable pests in other smaller scale insect families. The management of pest scale insects requires a combination of tactics including insecticide releases and biological control. Successful exam-ples of control often involve specific predators or parasitoids, but the fight against polyphagous invasive species may be impeded by a lack of knowledge about their systematics. Besides tradi-tional integrated pest management programs, other innovative and promising approaches are under investigation. IntroductIon Scale insects (order Hemiptera: suborder Sternorrhyncha: superfamily Coccoidea) are small sap-sucking insects with soft bodies, and there are about 8000 described spe-cies in over 30 families worldwide. They show a high number of very specific morphological, genetic, and bio-logical traits. Their sexual dimorphism is remarkable, with nymph-like adult females and two-winged adult males that lack functional mouthparts. Both the general reduction of female appendages and body segmentation and the produc-tion of wax by specialized glands make the (often sessile) females cryptic and hardly visible in the microhabitats of their host-plant. Thus the discovery of new infestations is often delayed until populations have proliferated or the plant symptoms are obvious. The first actions affecting the plant are sucking and ingestion of sap from virtually all sites on the plant, depending on scale species, as well as the physical penetration of stylets and injection of often toxic saliva into plant tissues. Most species of scale insects are phloem feeders (except Diaspididae that feed on parenchyma) and produce large amounts of honey-dew rich in sugars, which can be used by ants and lead to mutualistic relationships. This honeydew also facilitates infection of plants by fungi, sometimes causing severe secondary complications. Unlike other hemipteran groups (jumping plant-lice, leafhoppers, and planthoppers), the scale insects are rarely known as vectors of bacterial path-ogens or phytoplasmas. Only very few species of scale insects are involved in virus transmission, notably on grapevines and pineapples. The known and available natu-ral enemies for biological control include entomopatho-genic fungi, predators such as lady beetles (Coccinellidae) and larval gall midges (Cecidomyiidae), wasp parasitoids (mainly Encyrtidae), and others.