While stumbling around my patio in the predawn twilight last week, I came upon an unusual grub with an even more unusual mode of locomotion. Larvae of the Green June beetle have a pretty unique way of getting from place to place. Although Green June beetle grubs have well developed legs on their thorax, legs are not the primary mode of locomotion when grubs arise from the earth. These wiggly critters have a series of stout hairs on the upper surface of their back. To move above ground, the grub rolls on its back and with peristaltic motions it wriggles across the surface of the ground or my patio. The stout dorsal hairs contact the substrate and provide ample traction for surprisingly rapid movement.
The saga of the back-walking grub began back in the sweltering summer when its mother deposited eggs in soil. The flight patterns and buzzing sounds of Green June beetle adults are strongly reminiscent of large bumble bees. Perhaps, these behaviors are a clever way to ward off would-be predators that learned not to mess with buzzing, stinging insects. Green June beetles are part of a clan known as fruit and flower chafers or Cetoniinae. While most flying beetles spread their hard outer wings to fly, members of the Green June beetle clan simply lift their hardened outer wings, expand membranous hind wings, and take off.
Male beetles zoom in the morning as they search for mates. Females also fly low as they search for suitable places to deposit eggs in the soil. Once the female locates a favorable spot, she burrows several inches into the earth, makes a large sticky ball of soil and proteinaceous goop, and deposits eggs in it. Eggs hatch in a few days into small c-shaped white grubs. During the day the white grubs rest in a burrow underground, but at night they move to the surface of the earth to eat decaying organic matter. With some regularity they wind up on my patio or carport.
Soils with organic mulches and grass clippings like my miserable garden, and farm fields that received applications of manure are highly attractive to the egg-laying females and may be loaded with grubs. Most of the damage caused by grubs results as they move to the surface of the soil to feed. Their burrows can be the diameter of your thumb and small mounds of soil often surround the burrows. The disruption of soil caused by burrowing beetles creates problems on golf courses and in lawns. After feeding for several months near the surface of the earth, the grub, now an inch and a half long, burrows deeper underground to escape winter's bitter cold. In spring as temperatures warm, larvae return to the upper strata of the soil to resume feeding and complete development.
Unlike their more destructive relatives, Japanese beetles, Green June beetles are not serious pests of roses, lindens, or other plants. Their primary foods are thin-skinned fruits such as berries and grapes. I have also observed several adults congregating to eat fermenting exudates from a wound on a tree. Fresh fruit and fermenting beverages sound just fine on a warm summer’s day. Maybe these Green June beetles are just a bit smarter than we think.
Much of the information for this Bug of the Week came from Daniel Potter’s excellent reference book “Destructive Turfgrass Pests”, Ann Arbor Press, and the interesting article “Mimicry of Hymenoptera by Beetles with Unconventional Flight” by R.E. Silberglied and T. Eisner.
To learn more about Green June beetles on line, you can visit the following web sites: