Who's in Your Compost?
Larvae of the green fruit beetle are large grubs, and crawl on their backs instead of their legs. They help to decompose organic matter, and will not feed on live plants.
The Utah Plant Pest Diagnostic Lab received numerous calls this spring concerning white grubs in compost. In general, there can be many beneficial organisms present in compost, including centipedes, flies (maggots), predatory and other mites, rove beetles, ants, springtails, sowbugs, pseudoscorpions, earwigs, nematodes, earthworms and more. One of the largest organisms found in compost, however, is the larval stage of the green fruit beetle (Cotinis mutabilis).
The green fruit beetle is a scarab beetle and is related to the May/June beetles. Unlike some of its economically significant relatives, the green fruit beetle is not a major plant pest. However, the adult beetle can cause damage (usually minor) when they eat maturing and ripe thin-skinned soft fruit such as tomato, peach, plum, fig, grape, pear, blackberry, raspberry, apple, nectarine, and
The adult green fruit beetle will sometimes feed on the skin of soft fruits such as tomatoes, peach, and plum, but is not usually a pest of concern.
Cotinis larvae can be large, up to 1.5-2 inches in length! Larvae are white grubs that have 6 legs and roll into a “C” shape when disturbed. The larvae of green fruit beetle actually crawl on their backs instead of their legs. This behavior can help with field identification. The green fruit beetle overwinters in the larval stage. The adult beetle is a Japanese beetle look-alike, but it is much larger, about 1.25 inches long and metallic green in color. Adults are noisy fliers and are active in July and August.
If green fruit beetle larvae are found in small numbers, their presence can be considered beneficial, and they will not cause damage to garden plants. In large numbers, larvae can be hand-picked or screened out of compost before applying to the garden. The larvae can be added back into the remaining compost pile to aid in decomposition, left to desiccate, or fed to chickens, etc. If compost is heating properly, high temperatures will kill the larvae. Since the larvae are particularly fond of manure, non-composted manure could be removed to prevent populations from building up. Areas with manure present can be flooded for a minimum of 2 days to kill larvae. Frequent turning of compost can also increase larval mortality.
-Ryan Davis, Arthropod Diagnostician
Trautmann, N. 1996. Cornell Composting Science and Engineering: Invertebrates of the compost pile. Cornell Center for the Environment.
Solana Center for Environmental Innovation: Fresh Perspectives. 2013. Rotline: Are the white grubs in my bin safe to put in my garden?
Flint, M.L. 1998. Pests of the Garden and Small Farm, 2nd Edition. Statewide Integrated Pest Management Project, University of California, Division of Agriculture and Natural Resources Publication 3332.
Crahsnaw, W. 2004. Garden Insects of North America. Princeton University Press.