May beetles are a group of brown-to-black beetles that can emerge suddenly from lawns and grassy fields in early spring, especially following rain. Depending on the species, larvae (white grubs) will have fed on grass roots for several months, or even several years, before emerging as adults (Figure 1) to reproduce.
Some May beetle species do not feed, while others feed ravenously at night. Extensive damage can appear suddenly; however, the culprits hide during the day, leaving the cause a mystery. Defoliation is often most serious on trees in or near pastures or large grassy areas where thousands of the insects are produced.
Adults emerge in early May and are present for about a month. They prefer foliage from most of the common oaks, as well as birch, elm, hickory, and walnut. These insects may completely strip trees; entire leaves may be eaten or petioles may be cut so leaves drop to the ground. The beetles also often feed on (but rarely strip) ash, fruit trees, hackberry, locust, Lombardy poplar, maple, plum, and willow.
Large, healthy trees should be able to withstand moderate defoliation. New landscape trees are most vulnerable to attack. If damage is caught early, then a foliar spray of tree and shrub products containing any of these or similar active ingredients can be used to protect the remaining foliage: bifenthrin, carbaryl, cyfluthrin, malathion, or permethrin. These names can be found in the Active Ingredients box below the brand name on the product label.
By Lee Townsend, Extension Entomologist