Stethorus pusillus (Herbst, 1797)
Widely referred to in the literature as S. punctillum (Weise, 1891) but now accepted under the older name of pusillus, this very efficient predator of spider mites is known under various common names such as the ’Spider Mite Destroyer’ and is widely available commercially for use under glass to combat infestations of the hugely-destructive Red Spider Mite (Tetranychus urticae C. L. Koch, 1836) which is known to attack and seriously weaken almost any plant of commercial value. It was introduced into North America in 1955 to help control citrus and avocado mites in orchards in California but failed to become established, since that time it has been unintentionally introduced on several occasions and it is now established in southern Canada and the along the west coast and the north east of the United States. It was first produced in large numbers in the 1990s following breeding experiments in the United States and is now available worldwide for use by both commercial and domestic producers. The species is native to the Palaearctic region and occurs sporadically from Western Europe to the far east of Russia, China and Japan, it is absent from warmer areas, including central and South America, Africa and the Oriental and Australasian regions although specimens are very likely to occur along with horticultural produce almost anywhere. In Europe it is widespread and sporadically common; it extends into northern parts of Scandinavia and the UK where it is occurs throughout central and south east England. In the UK it is thought to be declining but because of its small size and concealed habits may well be under-recorded. The species occurs naturally in a wide range of habitats but is more usually associated with trees and shrubs in deciduous woodland, parkland and orchards, where it is sometimes abundant on fruit trees, and it may be common in hedgerows or on shrubs such as Hawthorn (Crateagus monogyna Jacq.) or Blackthorn (Prunus spinosa L.) in grassland or wasteland etc, and occasionally on Privet (Ligustrum ovalifolium Hassk.) in domestic gardens. Adults occur year-round, they overwinter among bark or leaf litter and are active over a very long season from early spring, peaking in abundance from June until September, they breed during spring and summer and both adults and larvae predate mites although they will also take small aphids and other insects and their early stages. Under good conditions the life cycle can be short (see below) but it is not known whether there is more than a single generation each year in temperate regions.
Stethorus pusillus 1
Stethorus pusillum 2
Under artificial conditions the entire life cycle from egg to adult takes about three weeks at 70F and the total lifespan is about 65 days. Mating occurs soon after the adults have fed and females will readily oviposit above and below leaves infested with spider mites, they lay up to 13 eggs each day in small batches and produce about 150 eggs although much higher numbers have been recorded. Larvae emerge within 5 days and begin to feed as soon as they find prey, they pass through 4 instars within two weeks or so and in this time may consume 240 mites, when fully-grown they move down the plant to pupate on the underside of leaves. Adults eclose between 6 to 8 days after pupation and will feed after a period of hardening and colouring, females generally need about a week before they are ready to lay eggs but this may be shorter if stimulated by an abundance of prey.
Adults may be sampled by beating or sweeping foliage but because of their very small size they must be looked for very carefully, finding signs of extensive spider mite damage and then carefully searching above and below leaves can be a good way of finding them in numbers, and taking samples of bark etc during the winter for extraction may also be productive.
1.3-1.5 mm. Broadly-oval and continuous in outline, body entirely shiny black and without obvious microsculpture, appendages variable from pale orange to dark brown, almost black with paler tarsi, mouthparts pale. Dorsal surface with pale creamy pubescence which tends to be rather erect and random on the forebody but smoothly directed to the apex and not forming any pattern on the elytra, ventral surface weakly convex and extremely-finely punctured and pubescent. Head flat and finely but not densely punctured between large oval eyes that are not emarginate anteriorly, antennae short, barely longer than the interocular distance, and gradually expanded to the apex, terminal maxillary palpomere strongly broadened to a wide truncate apex. Pronotum transverse and strongly convex, widest at distinct posterior angles and narrowed to projecting anterior angles, basal margin gradually produced to the centre, surface evenly convex and punctured as the head. Elytra evenly and finely punctured throughout. Distinguished from other tiny ladybirds by the colour, smoothly-rounded eyes and elytral pubescence which is directed back and does not form a pattern or lie at an angle against the suture-at least in the apical half.