Insects

Seen any Sawflies?

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By Dawn Pettinelli, UConn Home & Garden Education Center

Hibiscus sawfly larvae on hibiscus leaf. Photo by dmp2024

Gardeners know that amidst the promises of each new growing season come its challenges, usually in the form of weather or pests. A common insect pest this time of year are sawfly larvae, which feed on some of our favorite plants including roses, azaleas, perennial hibiscus, dogwoods, and some pines. If populations are high, significant damage can occur.

Sawfly adults are small, short-lived insects slightly resembling wasps to which they are related but their waists are generally thicker. Their ovipositor (egg-laying appendage) is somewhat saw-like in appearance, hence their common name. Eggs may be laid singly or in groups.

Larvae look like small caterpillars but there is a distinctive difference between caterpillars in the Lepidoptera family that transform into butterflies and moths, and sawfly larvae in the Hymenoptera family. Larvae in both families have numerous pairs of legs. From the head, the first three pairs are thoracic legs that correspond to an adult insects’ three pairs of legs. Count the number of fleshy leg pairs (pro-legs) after that. Caterpillars have 5 or less pairs while sawfly larvae have 6 or more.

Rose slug damage on rose leaves
Rose slug damage on rose leaves. Photo by dmp2024

Check your rose bushes for rose slugs, a common name for at least 2 species of sawflies whose larvae may be feeding on your plants. One is thought to be native to the U.S. and the other accidentally introduced from Europe. Both were noticed as pests in the Boston area over 100 years ago and do similar damage to rose foliage.

The small green caterpillar-like larvae that may be ½ to ¾ inch long depending on the species skeletonize the leaves and only the veins remain. They can feed for several weeks if not controlled and move to the undersides of the leaves during the day making them hard to spot. One or two generations may be present each season, again depending on species. Mature larvae will pupate in the soil below plants, where they overwinter and will reemerge the following spring.

Hibiscus sawflies feed on members of the mallow family with the most common one in this area being the large-flowering perennial hibiscus. The green larvae are small, and also can be found on the undersides of leaves where they often go undetected until large leaf areas are consumed.

There are at least 3 species of sawflies that feed on azaleas in this area. The small green larvae have tannish heads but are hard to spot at first as they feed on emerging foliage. Keep an eye out for foliar feeding at the edges of the leaves. The larvae are less than ½ inch in length and feed until late June or early July. There is only one generation per year and these larvae will also drop to the ground to pupate and overwinter in the soil below plants.

Pine sawfly species seem to prefer Mugo, Scots and red pines although they can feed on others. Some are inadvertently introduced pests like the European pine sawfly. Since they were first noticed around 1925, they have become widespread pests. The larvae may be up to an inch long and is grayish-green with mostly darker parallel stripes. Red-headed pine sawfly larvae have red heads as the name indicates but has a yellow body with black spots.

Pine sawfly adults typically lay groups of eggs so multiple larvae hatch and feed in groups. They have a very distinctive defense mechanism when disturbed. Most curiously they rear up and ‘wave’ as you wave your hand over them. This does discourage insect, avian and vertebrate predators, at least to some extent.

Dogwood sawfly females can lay more than 100 eggs on the underside of their preferred grey dogwood host plant. Larvae can consume the whole leaf as they mature leaving only the large midvein. Young larvae are translucent and yellowish but develop black spots and become covered with a white powdery coating as they mature. Their appearance can make a gardener think there are two species of insects attacking plants rather than one. When mature, the larvae seek out an overwintering location with rotted wood on the ground being preferred.

Regardless of which species of sawfly is attacking your plants, options for control include hand picking, insecticidal soap, Spinosad or other insecticides labeled for sawflies. While Bacillus thuringiensis (BT) can be used for caterpillars in the Lepidoptera family, like cabbageworms and Eastern tent caterpillars, it does not work on sawflies.

If you have questions on identification or control of sawfly larvae or any other gardening questions, feel free to contact us, toll-free, at the UConn Home & Garden Education Center at (877) 486-6271, visit our website at www.homegarden.cahnr.uconn.edu

 

Eastern Tent Caterpillars

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By Pamm Cooper, UConn Home & Garden Education Center

Eastern Tent Caterpillar Tents in a tree

Eastern tent caterpillars, Malacosoma Americanum, are native to North America and are noticed mainly because of the silken tents they build in the forks and crotches of trees in early spring. While most tents are found on black cherries, they can also be found on other trees such as apple and crabapple. They are readily visible since tents are constructed before leaves fully expand. There is only one generation per year.

These caterpillars are hairy and dark when small, and later instars have two yellow lines along the back. A white medial stripe goes along the sides of the body, and this is flanked by two yellow lines. There are blue blotches along its side as well. Bottom hairs on the body are rusty brown. Forest tent caterpillars are also hairy, but they do not make tents, and their bodies

are blue with white dots on the back. Handling should be done with gloves on as some people may have sensitivity to the hairs.

Egg masses are black, laid on small branches of host trees the previous year, and the egg mass rings around the branch. Sometimes they are mistaken for small black knot galls. 150-300 eggs may be in each egg mass. Caterpillars typically hatch as black cherry leaves begin to expand the next spring. Caterpillars feed on leaves and buds and rest communally by day in the nests they construct from silk.

While they can almost defoliate small trees, these caterpillars usually have completed their feeding stage in time for the trees to put out new leaves by early summer. The more tents there are on small trees, though, the greater number of caterpillars there are, so damage will increase.

While protected from many predators while inside the tents, there are birds that will tear the tents apart to feed on the caterpillars inside. Both the black-billed and the yellow-billed cuckoos are known for their ability to do this, and native vireos will also seek out and rip apart the tents.

Mature caterpillars leave the host plants in early summer in search of suitable sites to pupate within a silken cocoon they will spin. Moths emerge later that year. The moths have a fluffy light brown body, and the wings have three bands with the center being narrower than the outer bands and a lighter brown in color. The center band is bordered on both sides by thin, creamy bands.

Eastern Tent Caterpillar Tents in a tree

Occasionally, eastern tent caterpillars have outbreak years where large numbers of the tents can be seen on the same tree. Fresh buds may also be consumed if trees are severely defoliated. When found on residential properties, tents can be swept apart by rakes or brooms, exposing caterpillars to predators, or knocking them off the trees. If warranted, foliage can be sprayed with a biological product called Bacillis thuringiensis subspecies, Kurstaki, or BtK, which only harms caterpillars and not other insects. Caterpillars will ingest the product as they feed on the foliage.

Most healthy trees can tolerate severe defoliation from eastern tent caterpillars. If concerned, look for egg masses in the winter on trees that had been damaged the previous spring. Remove them, if possible, or crush them, or prune the branch off below where the egg mass is found. Or let them be if the trees appear to have recovered. Letting nature take its course with native insects on native trees can often benefit birds and other predators. Do not confuse them with the notorious gypsy moth caterpillars which do not make silken tents or nests of any kind.

If you have a question about tent caterpillars or other gardening topics, contact the UConn Home & Garden Education at (877) 486-6271 or www.homegarden.cahnr,uconn.edu or your local Cooperative Extension Center.