The cherry ermine moth (CEM), Yponomeuta padellus

INTRODUCTION:
Earlier this month (June 7) a tree in Olympia was found defoliated so severely that the genus of the tree was in question (see photo). Is it a Prunus or is it a Mallus? This of course is the first critical step of successful monitoring in an IPM program; to identify the host that the damage is found in, and then look at the type of damage caused.

After further investigation, the tree was determined to be a Prunus by nursery inspection staff with WSDA. After collecting larval samples, and raising them to pupation, Eric LaGasa, entomologist for WSDA confirmed the culprit is the cherry ermine moth, Yponomeuta padellus, not the more prevalent apple ermine moth (AEM).

This type of defoliation has been unrecorded (until now) by this lepidopterous pest in North America! Another first for the State of Washington (at least its not Whatcom County). In the late 1990’s the apple ermine moth caused considerable defoliation in crabapple. The two species cherry (Yponomeuta padellus) and apple (Yponomeuta malinella) ermine moth are very similar in biology, morphology and damage.

HISTORY:
In 1992, Northern Ireland had approximately 150,000 km (~95,000 miles) of hawthorn hedge, and the CEM was considered a serious environmental threat. Not only due to its intensive populations causing potentially severe defoliation but also the environmental repercussions of widespread use of an insecticide on this extensive growth of hedgerows. This prompted studies of several different parasitoids of the CEM in Europe. A little closer to home, the CEM was first discovered in North America in 1993, in British Columbia. In Washington, there CEM has only been found on hawthorn, and only in low numbers which did not cause economic loss.

IDENTIFICATION:
The moths of the genus Yponomeuta have front wings that are white with black dots. Adults have a wing span of approximately 10 mm (0.39 in). Full grown larvae are approximately 20 mm (0.78 in) in length, and are cream coloured with black heads and spotted lines.

BIOLOGY:

CEM Pupa Image Source: Eric LaGasa, WSDA

The life cycle of CEM starts with the female laying eggs in the fall on twigs and branches and then covering them with a sticky secretion making them difficult to see. The eggs hatch soon after, and the miniscule larvae immediately prepare to overwinter. These tiny first instar larvae are found under the egg mass laid by the female moth the previous summer. On the Pacific West Coast the larvae are present from late April to around the middle of June. While the larvae grow they begin to make small webs and feed gregariously on new leaves. As the larvae progressively get larger, so does the size of the web or nest that can soon envelope entire branches. The webbed feeding areas can often be confused with other species. They then pupate and the adults start appearing in the beginning of July and remain present until August. Adults are attracted to light, so if you have a cherry tree close to your back door you might see them flittering around. If you are lucky to see a pupa, the two-tone colouring (see photo) of lighter and darker brown of the CEM is a distinguishing characteristic of this species. In contrast, the AEM pupae are completely brown in colour. One generation occurs per year.

HOST:
The most obvious difference between the two ermine moths is that they have different hosts. The cherry ermine moth feeds on Prunus trees such as cherry, plum, and peach as well as mountain-ash, hawthorn, and other related trees, while the apple ermine moth feeds exclusively on apple and crabapple.

Ladybird beetle larva attacking CEM larva Image Source: Eric LaGasa, WSDA

BIOLOGICAL CONTROL:
Keep your eyes open for predators such as the ladybird beetle (see photo) and parasitoids (Ageniaspis fuscicollis was imported in the 1980’s to control AEM). In Europe there are several different parasitoids that parasitize the CEM, not only Hymenopteran (stingless micro-wasps) but Dipertan (flies) as well.

NOTES:
Nests can be confused with fall webworm and tent caterpillars. The main difference to look for is that fall webworm nests are quite a bit larger and occur later in the summer; tent caterpillars build pads made of silk on the tree trunks and large limbs. The nest of the ermine moth will have loosely gathered leaves that extend down the branches, the webbing is not as thick as that of the fall webworm.