Controlling scale insects in fruits

The incidence of scale insects as a problem in fruit crops has increased in recent years. Most reports have been of San Jose scale on apples and a combination of San Jose scale and lecanium scale on stone fruits.

San Jose scale overwinters as a juvenile under the waxy scale covering and becomes fully mature by late May (in southwest Michigan). At this time, males come out from under the scale covering and fly to females, which remain under the scale, to mate. After mating, the females produce live young, called crawlers, for about a six-week period through the end of June and early July. These crawlers move across limbs, fruit and foliage until they find an attractive place to settle and produce a new waxy scale covering. They will then insert their slender, thread-like mouthparts into the plant and suck the sap. In large numbers, they can significantly reduce plant health, resulting in economic damage. There is a second generation of San Jose scale beginning again in late July and August. Damage from this generation to fruit can render the crop unmarketable.

Lecanium scale is similar to San Jose scale, but different in several respects. They overwinter as fertilized females that mature in June with eggs hatching in July. The young crawlers move to the underside of leaves, where they settle and feed along the main veins. Infested leaves can be stunted and fruit remain undersized. They move back to twigs in late summer where they will overwinter. Winged males appear in late August to mate and there is only a single generation each year.

The crawler stage of San Jose scale will be emerging in August, or 1,700 growing degree days (GDD) base 50 degrees Fahrenheit, such that materials active on crawlers should be considered. Assail is a good option if your farm also has codling moths and apple maggots to control at this time. Tourismo is another option that controls codling moths. Warrior will show broad spectrum activity on insect pests, but as a pyrethroid will carry risk of mite flaring. The insect growth regulators (IGR) Esteem and Centaur are more selective and slower acting, but are effective on scale insects.

— By John Wise, Michigan State University Extension, Department of Entomology