KC Gardens

Insects, galls and sawflies

Oak gall

Tip of the Week

Insects! Boy, do we have insects. The cooler, wet spring has brought with it many insects. The good news, for the most part, is these pests do not need control, as they are only cosmetic, nuisance feeders. Read more to learn about these insects.

Oak Galls A number of tiny, non-stinging wasps, mites and flies cause abnormal growths to develop on the leaves, twigs or branches of oak trees. These galls can include growths that are round, spiny, flattened, elongated or star-shaped. There are hundreds of different types of galls, each of which is caused by a specific insect. Galls form in response to a chemical that the insect injects into the plant tissue. Mature females lay eggs that hatch into leg-less grubs. Galls form around them. Larvae feed, develop, and pupate inside these galls. Adults may emerge either the same season or may overwinter inside the gall, depending on the life history of that specific insect.

Generally, these gall insects do not cause significant damage to their hosts, though some of the leaf galls can cause enough deformity to make a tree unsightly. Also, severe infestations of twig galls can cause twig dieback or, rarely, tree death. However, just because a twig is covered with galls does not mean it is dead. I have seen twigs that looked like a solid mass of galls leaf out in the spring. Insecticide sprays applied when galls are noticed are ineffective because damage has already occurred. Also, larvae are unaffected because of the protection afforded by the gall. Insecticide sprays can kill emerging adult wasps and flies, but long periods of emergence and short residuals of most contact insecticides make this impractical. Stem and twig galls can be pruned if deemed to be practical and necessary. In short, this is a problem that is best ignored unless pruning is done to improve the appearance of the tree.

Sawfly Larvae on Ash Trees We have had reports for the last two years about a number of ash trees that have sawfly larvae feeding on the leaves. These sawfly larvae are a light green color with a broad, whitish stripe on the top side. In the middle of the whitish stripe there appears to be a darker green stripe that is actually the digestive tract of the insect. Though sawfly larvae resemble caterpillars, they have at least six pairs of “stub-like” prolegs behind the three pairs of true legs on their abdomen.

This insect is a sawfly larva, not a caterpillar, as they never have more than five pairs of prolegs. There are usually no detrimental effects to the health of the tree if nature is allowed to run its course. Even if all the leaves are eaten, it is early enough in the growing season for trees to put out a complete new set of leaves and still have enough time to make all the food reserves needed to survive the coming winter. However, if control is desired, a number of insecticides can be used for control including cyfluthrin (Bayer Lawn and Garden Multi-Insect Killer), malathion, esfenvalerate (Monterey Bug Buster II) and Sevin. An effective organic product is spinosad (Captain Jack’s Deadbug Brew; Fertilome Borer, Bagworm, Leafminer and Tent Caterpillar Spray). Horticultural oils and insecticidal soaps are also effective because of the soft skin of sawfly larvae.

The bottom line is, nature is full of interesting creatures, and, for the most part, should be marveled at and enjoyed, not killed.

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