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Insects of spring: What to watch for

Tree Talk: Track insect development this spring with degree days, and watch out for these problem pests.

With spring’s arrival, insects and mites make their way out of their winter dormant stage and become active. Insects that overwintered as eggs, larvae or pupae either hatch from the egg or complete their development and reach their adult stage.

Spring temperatures will dictate whether this process is fast or slow. Insects are cold-blooded and do not resume physiological activity until temperatures approach about 50 degrees F. A rather simple way to track insect development is by using degree days along with the 50-degree threshold temperature.

Degree days are determined by calculating the average temperature for a given 24-hour period and comparing it to the 50-degree developmental threshold temperature. If the average temperature is 51 degrees or greater, then we accumulate degree days; if it’s less than 50 degrees, we record a zero for that day. We do not subtract degree days. To track insect development, keep a running total of degree days.

So, let’s apply the degree day concept to some early-season insects that you may find on some of your woody plants.

One of the earliest critters to show up are eriophyid mites, coming between 50 and 75 degree days. These arachnids (related to spider and ticks) are sap-feeding animals and are common on trees such as bald cypress. They require magnification to see and may go unnoticed. Feeding damage causes the foliage to be off or dingy in color.

Remember, bald cypress is slow to leaf out in the spring, and it may appear the tree is dead — so do not assume it is mite damage. Examine the new foliage and use a hand lens or magnifying glass to look for mites. You can also hold a piece of paper or white cloth under the branch, give it a good rap, and the mites will drop to the paper where you can see them.

Mite infestations are not lethal for a tree, but they do rob the tree of water and nutrients, potentially leading to bigger problems later. Mites can be controlled by miticides, but unless there is a heavy infestation, chemical control is usually not required.

A second early-season sap feeder, at 100 to 200 degree days, is the honeylocust plant bug. This insect overwinters as an egg, with the eggs hatching just about the time honeylocust trees are breaking bud. The young plant bugs are a light green and very active, running up and down the twig and feeding on the newly emerged leaves. The nymphs continue to develop into adults. The insect has only one generation per year, with the adults gone by late June.

Damaged foliage will be present the rest of the season, but any new growth will appear normal. The young leaflets will be shriveled, distorted and malformed. If you are standing or walking under a heavily infested honeylocust tree, small bright green bugs may drop down on you, but they are harmless. They are only interested in plant sap of honeylocust trees. Some years, populations are heavier than others.

Another early-season pest, also at 100 to 200 degree days, that can be easily overlooked is the European pine sawfly. Sawfly eggs hatch fairly early in spring into small olive-green larvae with black heads that look like caterpillars. While most caterpillars turn into moths or butterflies, sawfly “caterpillars” eventually develop into a small wasp-like insect. They are called sawflies because the female has a saw-like ovipositor she uses to lay her eggs in the needles of conifer hosts.

Common hosts include Scots and mugo pines, common landscape species. The eggs are yellow and are usually laid in a line running the length of the needle. Upon hatching, the young larvae feed on the margins of the needle. As they grow and develop, they eventually consume the entire needle. Fortunately, they only eat last year’s needles.

On infested trees, you will see the new candle, and then a bare area below that where the larvae have been feeding. Sawflies are gregarious and like to feed in groups, so you may see a cluster of larvae feeding on a branch. It is important to control sawflies when they first appear, because conifers do not produce new needles, leaving bare spots and affecting the aesthetic qualities of the plant, particularly Christmas trees. Outbreaks typically occur in Christmas tree plantations, windbreaks and privacy plantings.

Sawflies can be controlled using chemical insecticides, or if you have just a few and the time, they can be picked off the plant and destroyed. On smaller plants, a good strong stream of water can dislodge them.

In summary, none of these sap-feeding or defoliating insects are fatal to forest or landscape trees, but keep in mind that young, newly transplanted trees or trees that are struggling should be treated to protect them from repeated defoliation. The leaves and needles are the food-making organs for the tree, and the food (i.e., money) produced by the plant is how it “pays the bills.” For specific recommendations on chemical insecticides, consult your local Extension office. 

Miller is a horticulture professor at Joliet Junior College in Joliet, Ill., and a senior research scientist in entomology at The Morton Arboretum in Lisle, Ill. Email your tree questions to him at fmiller@jjc.edu.

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