I’ve had quite a few questions about little black caterpillars on hazelnuts this week. These guys are gypsy moth caterpillars, which were quite a problem in many Ontario hazelnut orchards last summer. Gypsy moths overwinter as egg masses on tree trunks, fence posts, buildings and other sheltered locations.  The larvae hatch as temperatures warm in the spring and then disperse by producing silken threads that they use to ride the wind to new hosts. Egg hatch occurred in mid-May and the young caterpillars are now actively feeding in hazelnut orchards.

Early instar gypsy moth larvae feeding on hazelnut. Note the round holes typical of early feeding damage.

Newly hatched gypsy moth larvae are black or brown, about 4-6 mm long, and have long hairs.  Photo: J. Llewellyn, OMAFRA

If you see holes in your hazelnut leaves, turn them over and look for tiny, black fuzzy caterpillars. These will change in appearance as they grow, becoming grey with double rows of red and then blue spots running down their bodies.

Mature gypsy moth larvae. Note the double row of blue, then red dots running down the back, which distinguishes this insect from other caterpillars.

Gypsy moths have a wide host range including a wide variety of landscape trees including Quercus, Acer, Fagus, Tilia, Betula, and they definitely like hazelnut leaves. Gypsy moths tend to have boom and bust life cycles, building to huge numbers every few years before predators and pathogens catch up to them and their numbers crash to very low levels for several years.  Judging by numbers in hazelnuts and other landscape trees, this is definitely a boom year for gypsy moths.

The larvae will feed on leaves for several weeks before they pupate in early summer.  The good news is that the pupae and adults do not feed and there is only one generation of larvae per year, so after they pupate there will not be any further damage.  Unfortunately, until then the caterpillars feed pretty voraciously as they grow.

Mature, established hazelnut trees can tolerate some level of defoliation but caterpillar feeding can significantly defoliate young, establishing hazelnut trees, so control may be required. There is no established threshold for gypsy moth on hazelnuts, but thresholds for this insect on other crops in Ontario or from caterpillars on hazelnuts in other areas can be used as a general guide. In Ontario apples, a threshold of 12-15 larvae or damaged leaves per 100 terminals (examine 10 terminals/tree on 10 trees) is used for gypsy moth.  In Oregon hazelnuts, 20% infestation is used as a threshold for the winter moth caterpillar.  These can serve as a general guide to help determine whether control is warranted, but growers should use their experience and judgement of what their crop can tolerate.  Very young or newly planted hazelnuts may required control at lower caterpillar numbers due to the limited leaf area on these trees.

Gypsy moths can be handpicked and crushed if there are small numbers of larvae and small trees.  It is also possible to install a burlap skirt at the base of the tree to create a shady, protected area for larvae to hide during the day (this behaviour usually peaks near the end of May). These burlap skirts and underlying bark crevices daily (1-3 pm is best) need to be inspected daily and the larvae removed and destroyed. Sticky bands around trunks during the June/July flight period will help trap flightless females and keep them from laying eggs on the bark.

These cultural practices may not be practical in larger orchards. In these cases, sprays may be required.  A list of registered products on hazelnuts can be found in OMAFRA Publication 360E – Crop Protection Guide for Tree Nuts (http://www.omafra.gov.on.ca/english/crops/pub360/p360toc.htm) . Products applied for leafrollers in hazelnuts should also provide control of gypsy moth. At this time, the biological insecticide Bacillus thuringienesis (B.t., Dipel, Bioprotec, Xentari) can be quite effective especially in the first 2 weeks after larvae start to feed. This insecticide must be consumed by the larvae to be effective, so it should be applied close to the time that larvae are actively feeding. Good coverage of leaf surfaces is also important.