If you noticed small black caterpillars munching on flower buds or leaves on your trees last year, you weren’t alone. These were gypsy moth larva (Figure 1) which were quite a problem across the province – and may be an even bigger issue this spring.
Gypsy Moth Boom
The Ontario Ministry of Natural Resources and Forestry (MNRF) reported that defoliation caused by gypsy moth in Ontario increased from approximately 47,000 ha in 2019 to nearly 600,000 ha in 2020 (Figure 2), beating the original record of 350,000 ha from 1991.
Egg mass surveys conducted over this last fall and winter suggest that this year, defoliation is likely to be severe in many regions (Figure 3).
Any natural assistance on reducing moth populations by climate, fungus, virus and parasites will not be known until later this spring. More information on these surveys can be found on the MNRF website.
While gypsy moth have a wide host range of over 300 species of trees, they tend to prefer maple, beech, linden, birch, oak, poplar, Prunus and willow. Outbreaks occur about every 7 – 10 years and have a boom/bust life cycle. In other words, populations build to large numbers every few years until their predators and/or pathogens catch up, causing numbers to crash again. What we’re seeing right now is definitely a boom for gypsy moth!
Activity in the Orchard
Gypsy moths overwinter as spongy egg masses on tree trunks (Figure 4), fence posts, buildings and other sheltered locations. Typically, eggs hatch mid-May. Though with the early spring this year, activity may start sooner.
Once hatched, larvae disperse to new foliage by wind. Initially, larva feed during the day which is why monitoring for spring-feeding caterpillars is so important. As the larvae mature, feeding switches mainly to night and can often delay detection of a problem if not caught early.
If you see holes in leaves, turn them over and look for tiny, gray or black caterpillars with wispy hairs (Figure 5). These will change in appearance as they grow, developing distinct spots that are unique from other hairy caterpillars. Starting just after the head, there are five pairs of blue spots followed by six pairs of red spots down the back.
Larva will feed on leaves for several weeks before pupating. Most damaging activity is only present in orchards until July. 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, that early season larva can feed pretty voraciously until then.
Monitor For Damage
Despite the trees’ ability to produce a new crop of leaves over the summer, the damage can cause significant impacts to growth. This is an even greater issue with young plantings. Defoliation also makes the trees more susceptible to other issues like secondary pests, drought and winter injury.
Take note of gypsy moth activity during regular springtime orchard monitoring. Between tight cluster to petal fall, check 5 terminal shoots and 5 fruit buds from 10 trees (50 terminals and 50 fruit buds in total) for signs of caterpillar feeding activity. After petal fall, monitor 10 terminals on 10 trees (100 terminals total) until mid-summer. An insecticide is generally recommended when the action threshold of 12-15 larvae per 100 terminals and fruit buds is observed. This threshold 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. Higher density or young orchard blocks may required control at lower moth populations due to the limited leaf area on these trees.
If there are a small number of larva or limited area affected, there are a number of cultural controls that can be done (Figure 6):
- Gypsy moth can be handpicked from the tree and crushed.
- Wrapping the base of the tree with burlap when larvae are active (late May) creates a sheltered area for larvae to hide during the day and is also a management option for smaller blocks.
- However, be sure to check these burlap wraps and underlying bark crevices daily (1-3 pm is best) and destroy any larvae found.
- Sticky bands can also be placed around trunks during the June/July flight period to help trap flightless females and keep them from laying eggs on the bark.
Obviously, these cultural strategies may not be practical in larger orchards. Products applied for leafrollers in apples such as Bacillus thuringienesis (Bt), e.g., Dipel, Bioprotec, Foray and Xentari should also provide control of gypsy moth. Other products listed under spring-feeding caterpillars in Publication 360A, Crop Protection Guide for Apples may be registered for this pest, including some pyrethroids (Group 3) and Cormoran (Group 4A+15). Always refer to the label for registered pests. For best efficay, apply within the first few weeks after larval emergence and close to the time larvae are actively feeding to ensure exposure. Good coverage of leaf surfaces is also important.
Aside from the options mentioned above, many predators (eg., birds, ground beetles, stink bugs, assassin bugs, spiders) and parasitic insects can attack spring-feeding caterpillars. Encourage natural enemies by applying insecticides only if an action threshold is reached and select materials that have lower toxicity to non-target organisms. See Table 3- 6. Toxicity of Pesticides to Honeybees and Mite/Aphid Predators in Publication 360A, Crop Protection Guide for Apples for more information. A number of fungal and viral diseases also impact gypsy moth, particularly during warm, wet springs.
A great resource on gypsy moth in Ontario landscape including high quality photos for identification is this video from OMAFRA’s Nursery & Landscape Specialist, Jennifer Llewellyn.