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EB1526
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| The apple ermine moth, Yponomeuta malinellus, is an introduced pest of Eurasian origin. It was first found in northwestern Washington in the summer of 1985. By 1989, it had been detected in Whatcom, Skagit, San Juan, Snohomish, Island, and King counties and in the lower Frazer River Valley in British Columbia. | ||
Damage |
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| The apple ermine moth feeds exclusively on apple. If tents and larvae are numerous enough, serious defoliation can occur. In some cases, almost complete defoliation has been observed in May or June. In addition to the feeding damage, another problem has emerged. As an introduced (exotic) pest occurring only in the Northwest, apple ermine moth is a threat to the nursery industry—exporters of large quantities of apple planting stock. Nurseries in the infested areas are under quarantine and are required to provide shipping stock completely free from apple ermine moth. | ||
Biology |
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Apple ermine moths overwinter as young larvae underneath the egg mass, which the female deposited on tree bark the previous summer. Some bark feeding may occur under this egg mass at times. In mid-April larvae begin to emerge from this protective covering (or hibernaculum) and move into nearby developing leaves. At first they mine the leaves, but towards the end of bloom they begin to feed more as tent caterpillars do within a communal web. The webs are extended to engulf more leaves and can be as large as a tennis ball. Unlike tent caterpillars, apple ermine moths will make several tennis ball-sized tents during larval development—not one large community tent. Feeding in this fashion continues till early or mid-June, when pupation begins. Pupation is striking in this pest, as it is also a communal event. Pupating caterpillars arrange themselves in a tightly packed cluster, neatly lined up like cord wood. Adult moths begin emerging in late June, and females lay eggs from July into September. Females lay 10 to 80 eggs to a cluster in overlapping rows like shingles. The egg mass forms a flattened, slightly convex oval about 1/8 inch in diameter. Freshly laid egg masses are light yellow in color, changing to bright red within about 2 weeks. Aging masses then fade to a cryptic gray, which is difficult to see on apple tree bark. Larvae vary from gray to dark greenish or cream colored with pronounced spots along the sides. At maturity, larvae are about 1/4 to 3/8 inch long. Pupae are contained within grayish white cocoons that are suspended in clusters within the feeding web. Adults have striking silvery white forewings with rows of small black spots. The wingspan of the moth approaches 3/4 inch. |
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| Apple ermine moth adults, upper left. Apple ermine moth egg mass at least a few weeks old {color varies from time eggs are laid to several weeks later), upper center. Mature apple ermine moth larva, upper right. Apple ermine moth tent and larval damage, lower left. Pupal clustering of apple ermine moth, lower right. | ||
Control |
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When tents are limited in number and easily accessible, they may be physically destroyed. May is the best time for this. Most chemicals registered for use on apples for the control of tent caterpillars or leafrollers should give adequate control. The bacterial insecticide Bacillus thuringiensis gives excellent control if it is applied to actively feeding caterpillars (April-May). Because this is an introduced pest, natural control is not yet significant. Since the discovery of apple ermine moth, however, the Washington State Department of Agriculture (WSDA) and the United States Department of Agriculture/ Agricultural Research Service have begun efforts to secure foreign parasites to control and slow spread of the pest. |
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