Defeat the defoliators

Have you ever been caught off-guard by caterpillars? I don’t mean that a gang of caterpillars jumped from behind a tree and scared the pants off you. I mean one week trees look fine and the next they are defoliated. Rapid growth and feeding make caterpillars important and challenging pests of trees in nurseries and landscapes. There are dozens of caterpillar pests of woody trees and shrubs, but in this article I cover the two most common pests of oaks and maples, which are two of the most common trees.

Both species are in the family Saturniidae. This family contains many of the world’s largest and most beautiful moths characterized by rings or eyespots on their wings that resemble Saturn. Orangestriped oakworms (Anisota senatoria) are common throughout the eastern U.S. and west to the Great Plains. They typically have one generation per year. Adults are 1-2 inches long and orange to pink with a stark white spot on each forewing. Female moths lay clusters of yellow eggs on the bottom of oak leaves in mid to late summer. Eggs hatch in a week or two then tiny young caterpillars feed in groups beneath leaves.

This is the sneaky part about caterpillars. They start small and are hard to notice, just skeletonizing leaves. However, they grow quickly and increase their body weight and feeding by hundreds of times. Orangestriped oakworm larvae become black with increasingly noticeable orange stripes. Larvae grow more than 2 inches long and consume entire leaves, leaving only the toughest midveins. They also become less gregarious and spread throughout a tree or groups of trees in search of new leaves.

Orangestriped oakworms, as you would expect, feed primarily on oaks. Red oaks, scarlet oaks, pin oaks, and willow oaks are preferred over white oaks, chestnut oaks and others for feeding and oviposition. Maples and other trees can be damaged by orangestriped oakworms, particularly if they are near a heavy infestation on oaks.

Greenstriped mapleworm, Dryocampa rubicunda, has many similarities to orangestriped oakworm. It is also found throughout the Eastern United States. Adults are called rosey maple moths. They are bright pink and yellow and about an inch long. They lay eggs in early summer and tiny yellow caterpillars feed gregariously like their orangestriped cousins. These grow into 2-inch green caterpillars with black horns. In just 30 days they are full grown and ready to pupate. However, unlike orangestriped oakworms, a second generation of moths will emerge from these pupae in a couple weeks to lay eggs again. Greenstriped mapleworm can have two to three generations per year in warm regions.

Caterpillars damage trees by eating leaves. Hundreds of caterpillar species feed on oaks and maples but only a couple are pests that cause noticeable damage. Even feeding by pests, like oakworms and mapleworms, may be limited to a single branch that could go without notice. However, sometimes and in some places, outbreaks occur in which many trees are defoliated. Sometimes because caterpillar outbreaks do not occur every year. Some places because certain nursery and landscape situations are more prone to damage than others.

It is hard to predict the years when caterpillar outbreaks will occur. The places where outbreaks occur are easier to predict. Most pests become more abundant and damaging when lots of their host plants are grown together under unnatural conditions. Thus, caterpillar damage can be worse in nurseries where many oak or maple trees are grown in tight quarters. Severe defoliation also occurs on urban trees where whole neighborhoods can be lined with one species of oak or maple growing in hot dry conditions.

These nurseries and urban street plantings make it easy for female moths (and other pests) to find their preferred host tree to lay eggs. Once the eggs hatch in these situations it is easy for caterpillars to move from tree to tree eating leaves. Presumably, if you are reading this you manage trees in nurseries or landscapes and may have to deal with orangestriped oakworms or greenstriped mapleworms. So, what do you do?

Scouting and control

Luckily, considerable research was conducted at Virginia Tech to develop integrated pest management tactics for orangestriped oakworm. The first thing you can do is look for moths and eggs in midsummer. In central North Carolina, moths turn up at porch lights and start laying eggs in late July. Inspect trees to find the masses of yellow eggs. I know this seems like finding a needle in a haystack, but each egg mass has up to 700 eggs, so it is worth finding them. Focus on the trees most likely to be infested, which means trees in groups or stressful conditions and trees that were infested the previous year. They often hit the same locations year after year because they pupate in soil near the trees where they feed.

Eggs are laid on the undersides of leaves so looking up from below you can scan for the yellow masses. Trees are pretty tough and can tolerate around 25 percent defoliation, especially late in the season, without reducing vigor. Citizen complaints also increase above 25 percent defoliation, so this has been established as a reasonable aesthetic injury level for orangestriped oakworm. Twenty-five percent damage can occur with as little as one egg mass on trees up to 18 feet or nine on larger trees around 40 feet.

Unfortunately, similar work has not been conducted to determine aesthetic injury levels and egg mass thresholds for greenstriped mapleworm or many other pests. Use orangestriped oakworm guidelines, but be sure to look for eggs earlier in the season and watch for multiple generations.

Small patches of defoliation are easier to see than egg masses. Each group of caterpillars came from one egg mass, so you can gauge how many egg masses were present and the potential for damage. But don’t get caught off guard. Defoliation happens fast, so a few nibbled leaves could become severe defoliation in a week or two.

In many cases caterpillar infestations are small and do not cause much damage. Even trees that are entirely defoliated will leaf out again and be fine unless it happens every year. In nurseries, mid-summer damage may not be a problem if trees won’t be sold and planted until fall. If an early infestation is confined to a branch or two a little pruning might be all you need. For high-value trees and those with yearly infestations, a variety of insecticides are available that can be applied to the foliage or even injected into the trunk. The most important thing is to monitor trees to catch problems early, so you are not caught off guard when a tree goes bare.

Steve Frank is Professor and Extension Specialist in the Department Entomology and Plant Pathology at North Carolina State University. You can reach him at sdfrank@ncsu.edu. For more information on nursery and landscape IPM visit http://ecoipm.org.