Increased incidence of scale insect pests on tree fruits

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

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. It is thought that this increase in pest pressure is a result of the hot and dry climatic conditions that have been prevalent over the last few years, and well as the loss of some conventional insecticides that were historically used for control.

The 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 August. Damage from this generation to fruit can render the crop unmarketable.

The lecanium scale is similar to San Jose scale, but different in several respects. They overwinter as fertilized females that mature in June, eggs then 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.

Table 1. Insecticide options for summer control of scale pests, targeting the crawler stage.

Compound Chemical class Mite flaring potential **
Superior Oil Oil L
Warrior, Proaxis, Baythroid XL Pyrethroid H
Assail, Calypso, Provado Neonicotinoid
Esteem IGR (juvenoid) L

** Mite Flaring Potential; L = low, M = moderate, H = high.

Dr. Wise's work is funded in part by MSU's AgBioResearch.

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