Stethorus punctum (LeConte)

I. Introduction: This lady beetle is usually the most important and frequent predator of spider mites observed in fruit orchards throughout the mid-Atlantic region. It is native to North America and was first identified in 1852. Research conducted annually since 1967 in Pennsylvania has provided much of the basic biology and management information for using this predator as the cornerstone of integrated mite management programs for deciduous fruit crops.

II. Hosts: Stethorus is strictly a predator of plant-feeding mites, particularly the spider mites important to this area, the European red mite, Panonychus ulmi (Koch) and twospotted spider mite, Tetranychus urticae Koch. Studies have indicated that areas in the orchard having an average of at least 2-5 mites per leaf are necessary to keep Stethorus active, and that from 8 to 10 mites per leaf are needed in these "pockets" to encourage the beetle to reproduce. The beetles feed on all stages of mites, and the adult can consume approximately 9 motile mites per hour, or about 75 to 100 motile mites per day. A third and fourth instar larva can consume up to 8 mites per hour.

III. Description: Stethorus adults are oval, convex, uniformly shiny black, and covered with sparse, fine yellowish to white hairs. The adults reach a length of 1/16 inch (1.4 mm) The adults are very active when in fruit trees and if disturbed will often fall to the ground. They are good fliers and thus tend to concentrate where prey is plentiful and to disappear when the mite population becomes low. The eggs are very small in size (1/64th inch [0.37 mm]), pale white, and oval. They become blackish just before the larva emerges. The eggs are laid singly on their sides, with 1 to 10 per leaf depending on mite density. Most of the eggs are laid close to the primary veins of the leaf and adhere tightly, with 95 percent on the under surface of the leaf and 5 percent on the upper surface. After an approximate 5-day developmental period, the larva emerges from the egg and starts to feed on all stages of mites present. The larva is gray to blackish and has many long-branched hairs and black patches. They have 13 segments exclusive of the head. As the larva matures it becomes reddish, at first on the edges of the body, and just prior to pupation the entire larva turns reddish. The larva passes through four larval stages and pupated in about 12 days. Stethorus pupae are uniformly black, small, flattened, and somewhat pointed on the posterior end. With magnification, lines representing abdominal segments can be seen. The wing pads are prominent and the entire body is covered with yellow hairs. For a short period after it is formed the pupa is reddish-orange. The pupal stage lasts an average of 5 days. The emerging adult is also reddish-orange during the first few hours after emergence from the pupal case before turning black.

IV. Biology: Stethorus overwinters as an adult beneath the trash cover ("duff") under fruit trees and in other protected habitats near the orchard. Stethorus adults can be found in the leaf litter immediately surrounding the trunks of fruit trees, in large numbers along the herbicide strip, and occasionally in the drive row. Recent research indicates the distribution of Stethorus to be closely associated with the leaf litter distribution in the orchard. The number of Stethorus overwintering outside the orchard is unknown, but they have been found along fence rows and adjacent wooded areas. It is advisable not to disturb the area in the herbicide strip near the trunk of the tree from November 1 to mid-April. Adults begin to emerge at the tight cluster stage of apple development with peak emergence from the pink to bloom stage. Adult emergence is usually complete by the petal fall stage. The adults remain active in the orchard until September to late October.

During early May females begin to lay eggs on the leaves of fruit trees. Egg laying occurs from May to mid-August. The peak periods of larval activity are dependent upon mite density, but in south-central Pennsylvania they occur during mid- to late May, late June to early July, and mid-August. After feeding for an approximate 12-day period the fourth-stage larva fastens itself to the leaf and remains there in a motionless state for 24 to 48 hours before pupation. Although pupae are constantly in the trees, the peak pupal periods are late May, early to mid-July, and late August.

Stethorus produces three generations per year in south-central Pennsylvania. The average period from the time the egg is laid to the appearance of the adult is 23 days. The adults feed for an average of 25 days before beginning to lay eggs. This time lag between emergence and egg laying is of little consequence because there is such an overlapping of active adults in the trees at all times.

V. Injury: This insect does not injure the fruit or foliage.

from a chapter in the Mid-Atlantic Orchard Mnitoring Guide, entitled Mite Predators,
by L.A. Hull and R. L. Horsburgh

See Cornell factsheet.
E-mail to: Douglas G. Pfeiffer