Gail

Thanks so much for reaching out to the UVM Extension Master Gardener Program Volunteers. I’ve broken down my response concerning the Red Osier Dogwood and then the River Birch. Hoping you find the following information helpful.

Sawfly Larvae (Red Osier Dogwood invader)

Adult sawflies are non-stinging, wasp-like insects that usually go unnoticed in the garden. However, sawfly larvae can cause significant damage to landscape plants. Immature sawfly larvae look very much like caterpillars, but they have a few distinct differences. Sawfly larvae have 6-8 pairs of prolegs (the rear legs attached to the abdomen of the insect), while caterpillars only have 2-5. Additionally, sawflies lack the hooks (crochets) found at the end of caterpillar prolegs, and most have bodies that are hairless.

Sawflies generally attack a small range of host plants, and you can narrow down which species you are dealing with based on what type of plant it is eating. Dogwood sawflies feed on the leaves of several shrubby dogwood species. The larvae have chewing mouthparts that create sizable holes in leaves. Dogwoods that have large chunks missing from their leaves are probably host to at least a few sawfly larvae.

Dogwood sawflies can create significant defoliation, largely because they feed in groups. Unchecked, heavy populations of dogwood sawflies can cause significant defoliation.

Adult dogwood sawflies emerge from late May through July and lay eggs on dogwood leaves. As the eggs hatch, the larvae feed on foliage between July and September. Dogwood sawflies go through several stages of development, or instars, before they molt into adults. In the second life stage, the larvae appear white and slightly fuzzy due to a waxy material they exude. When they aren’t actively feeding, the larvae will curl up on themselves and look like bird droppings at a quick glance. As the larvae continue to develop, they lose their white coating and become greenish yellow with black spots. By the time they are fully mature, dogwood sawfly larvae are about an inch long.

Controls: Since sawflies can cause significant aesthetic damage and have the potential to reduce plant vigor if left unchecked, monitoring for them in the spring is essential.

Small numbers of dogwood sawflies can be hand-picked from leaves and either squished or drowned in a bucket of soapy water. Heavier infestations may warrant the application of a low-impact, registered insecticide.

If you choose to use an organic insecticide, please work with your local garden center and follow all prescribed directions. Wear all personal protective equipment and spray when it is least harmful to other insects and birds. Spraying very early in the morning or late in the evening, when pollinators are inactive is always best.

Remember, insecticides tend to be most effective against young, small larvae that are half their full-grown size or less. Large larvae that are nearing maturity aren’t very susceptible to insecticides and should be hand-picked instead.

When working around dogwoods, it is a good idea to wear gloves and long sleeves because the leaves of some species can give gardeners an itchy rash. Whether hand-picking larvae or spraying, keep your skin covered to avoid brushing up against foliage.

Sawflies:

https://extension.umn.edu/yard-and-garden-insects/sawflies

https://hort.extension.wisc.edu/articles/sawflies/

https://extension.unh.edu/blog/2020/07/what-eating-my-dogwood-leaves)

Hickory Tussock Moth Caterpillars (River Birch invader)

You were spot on calling out the Hickory Tussock Moth Caterpillar on the River or Black Birch (Betula nigra). The caterpillars feed on a wide variety of hardwood trees, including hickory, pecan, walnut, American hornbeam, ash, elm, oak and willow, birch, basswood, to name only a few. The Hickory Tussock (Lophocampa caryae) is a type of tiger moth. The caterpillars are visually distinctive, with tufts of white and black hairs (setae) and black warts on their bodies. Their bristle-like hairs can cause a rash in sensitive individuals.

Because hickory tussock moths are a native species, they are usually controlled by a host of natural enemies. When outbreaks do occur, most defoliation occurs late in the season, so the caterpillars do not generally cause long-term damage to the affected tree.

If control is warranted or desired, the best method will depend on the life stage of the caterpillars. If egg masses or aggregations of young caterpillars are discovered, the effected leaves or branches can be pruned out. The eggs/caterpillars can be moved to a different part of the property where their feeding can be better tolerated, or they can be killed by submerging them in a bucket of soapy water.

Older, larger caterpillars, may be individually handpicked and killed in a bucket of soapy water (if hand picking, be sure to wear gloves). But again, by the time older caterpillars are present, it is unlikely that their feeding will damage the long-term health of the affected tree, so control is rarely warranted.

Tussock Moth Caterpillars:

https://extension.psu.edu/hickory-tussock-moth-caterpillar

https://extension.umaine.edu/home-and-garden-ipm/fact-sheets/common-name-listing/tussock-moth-caterpillars/

https://extension.umaine.edu/home-and-garden-ipm/fact-sheets/common-name-listing/hickory-tussock-moth/