Not all white grubs are bad white grubs. The ones we have been seeing at our office lately have been good grubs. Grubs that are doing you a favor by breaking down the cellulose in your gardens and potted plants turning it into the nutrients needed by your plants. These grubs are not the ones that feed on the roots or crowns of your plants and do not need to be nor should they be killed.
These grubs are the larval stage of the Bumble Flower Beetle (Euphoria inda) so called due to the buzzing sound they make when the adults fly. The adults emerge in the fall and feed on overripe fruit as well as nectar and goo that oozes from cuts and abrasions on many plants. The adults overwinter in the soil. You will see them flying again in the spring when they search out decaying wood, piles of manure and other rotten plant debris in which they lay their eggs.
If you conduct a Google or Bing search on white grubs in compost, you most likely won’t find many references to the Bumble Flower Beetle. The gardening forums often refer to the white grubs in compost as Japanese beetle larva and recommend you kill them to prevent them from moving into your lawn or onto your roses.
Why would you want to poison your compost with an insecticide to control white grubs that are not causing any harm and are actually beneficial? Even some of the white grubs found in your gardens are there feeding on the compost and manures you added last fall or this spring and not feeding on your plants. Like the Bumble Flower Beetle most other scarab beetles in Colorado are beneficial and help recycle nutrients in organic matter, such as dung and kitchen waste. Some of the posts on garden forums even recommend the use of Milky Spore Disease to kill these white grubs even though this material is not an effective control option for the scarabs we have in Colorado on this side of the continental divide. Milky spore disease is, however, an effective control option for Japanese beetle, a scarab beetle that is problematic in eastern Colorado. The only population of Japanese beetles that did occur in western Colorado in the town of Palisade was eradicated.
Take the time to look at the last abdominal segment of the white grub so you don’t mistake this beneficial white grub for a damaging one. Ohio State University has an excellent color publication on the various white grubs and their hind ends. To use this publication (http://www.oardc.ohio-state.edu/atru/research/grubs.pdf) you will need a hand lens. Pick up the white grub, flip it over on its back and check its butt. You are looking for the arrangement of spines and hairs on the rastar, the last abdominal segment. These spines and hairs form a distinctive pattern on the rastar of all white grubs other than the Bumble Flower Beetle. If you are squeamish and would like someone else to identify the white grubs you have in your compost or manure pile take them to your local Colorado State University Extension office and have them identified.
This white grub is also common in container grown plants feeding on wood chips, bark mulch, peat moss, and other dead organic matter. They do not feed on the roots of the plants growing in pots but have been shown to move nutrients throughout the container as they feed and move through the planting medium.
Take care when treating so-called insect pests. Some are very beneficial. When you spray for a pest without first getting it identified as a destructive pest you are doing more harm than good. Treat your environment with care!
I have to disagree with this post. Bumble flower beetles are thrashing my heirloom tomatoes, boring into my mammoth sunflowers and the stems. They did not wait for anything to be rotting or fermenting. They appear to be sucking the sap out of the sunflowers and diving head first into my heirloom tomatoes.
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