Scale Insect Control After the Methidathion Ban

Scale insects (also called armoured or soft scales) are a group of small, mostly sessile insects of the order Homoptera, oval, linear or oyster-shaped. Common species include the globose scale, arrowhead scale, cottony cushion scale, camphor round scale, chaff scale, Korean globose scale, mulberry white scale and long-tailed mealybug.

Scale-insect larvae have short legs: the young nymphs can move and feed, but as they grow the legs degenerate. The sexes are separate. Females are wingless; males have one pair of membranous forewings and hindwings transformed into halteres. They have strong reproductive capacity, producing 2–3 generations a year. Eggs hatch into nymphs which, after a short period of crawling, settle into a fixed life and form the scale covering.

Scale insects cause serious damage to fruit trees. For many years they have run rampant over large areas of pepper orchards, fruit orchards, persimmon orchards and jujube orchards; where growers control them poorly, losses are heavy. They feed on sap, and at high population densities damage leaves, twigs and fruit, causing leaf yellowing, withered shoots and declining tree vigour. Scale insects also secrete a substance that often leaves a white film over the damaged tissue, causing water and nutrient deficits and leading to dieback and death of the tree. In addition, their secretions can induce sooty mould, causing economic loss.

The peak period for scale insects is the hot, humid summer weather, with the most severe damage from late May to early June. The main difficulty in control is that the insects have a fairly hard shell and a thick waxy layer with strong resistance to chemicals, so ordinary products struggle to penetrate the body and control is relatively difficult. Once an outbreak occurs, it is hard to clean up completely.

For a long time growers used methidathion to control scale insects. Methidathion is broad-spectrum and fast-acting, is especially effective against scale insects and is economical, and was widely used on fruit orchards and vegetables. However, owing to its high toxicity, in 2015 the Ministry of Agriculture revoked the registration of methidathion on citrus and other fruit and vegetable crops. After methidathion was banned, effective scale-insect control products on the market became extremely scarce.

At present, the active ingredients commonly used on the market for scale-insect control are mainly spirotetramat, buprofezin and chlorpyrifos. Because scale insects have a waxy shell, the best products are those with strong penetration and systemic activity, such as spirotetramat. In the north, scale-insect eggs begin to hatch in April–May, and in the south in March–April; contact insecticides also work well at the peak of egg hatching. Imidacloprid has systemic activity and chlorpyrifos has good fumigant action, so both perform well against scale insects. In addition, tolfenpyrad also gives good control.

In China, scale insects occur heavily in the southern markets of Jiangxi and Fujian, concentrated on various cash crops such as fruit trees. Although the control market has declined somewhat with the fall in the number of ageing fruit trees, the ban on the highly toxic methidathion has created an opportunity for low-toxicity scale-insect control products. The domestic scale-insect control market therefore has a certain development potential.