Apple Ermine Moth

Apple Ermine Moth ,(Yponomeuta malinellus) is an insect pest of apple trees. The larvae can cause severe defoliation of apple trees. Y. malinellus larvae at first mine leaves, and then feed gregariously in a silken web in May and early June.

The Apple Ermine Moth (Yponomeuta malinellus) attacks apple and crabapple. Surveys are conducted because of the presence of potential host material in Wyoming, potential introduction pathways from surrounding states, and the ability of this pest to be transported in nursery stock. It has been found in the US in Washington and Oregon. To date, it has not been found in Wyoming. Eighty-four total traps have been placed throughout twenty-three counties in Wyoming.

The Wyoming Apple Ermine Moth Survey is conducted by the Wyoming Pest Detection / CAPS Program. A sticky trap with pheromone attractant is placed on or near the host plant from May until August. The traps are monitored for suspects and serviced once a month. If suspects are found they are sent to the University of Wyoming for further identification.

Life Cycle: Apple ermine moths overwinter as young larvae underneath egg masses that the females deposited on tree bark the previous summer. The larvae sometimes feed on bark under the egg mass. In early spring, larvae emerge from the protective covering, or hibernaculum, and move to nearby developing leaves. At first, they mine the leaves, but towards the end of the bloom period they begin to feed within communal webs, like tent caterpillars.

The webs expand to engulf more and more leaves and can be as large as a tennis ball. Apple ermine moth caterpillars may spin several tents in each tree, whereas the tent caterpillar may produce one or very few large ones.

The larvae continue feeding in this fashion until June, when they pupate. Pupating caterpillars line themselves up neatly in tightly packed clusters. Adult moths begin to emerge in June, and females lay eggs from July into September.

Damage: 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.

Monitoring : The best way to detect apple ermine moth is to trap adults. Bait wing traps with the commercially available apple ermine moth sex pheromone and place shoulder high in apple or crab apple trees. Check every 2 weeks to avoid damage to the wing pattern by stickum. Change the lure every six weeks or more often in hot weather. The lure is effective only within about 200 feet.

In May and June you can see webs containing larvae and pupae. The webs are about the size of a tennis ball or smaller and can be difficult to detect without close scrutiny, especially in large trees. Look for a gold cast on leaves that have been mined within the web. Do not confuse these webs with those of the tent caterpillar, which get much larger and extend along the branch. Searching for overwintering egg masses on the bark during fall and winter is not recommended, as the color of the hibernaculum blends in well with the bark.

Controls: 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).

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