How to Identify Grubs in Compost Bins
The perfect bin for your property has been delivered or built and set in place, and your compost has been layered with the necessary green and brown organic matter. You begin to dig down into the pile to turn it with a pitch fork or spade and notice some inserts. You wonder if they're grubs--the larvae stage of a beetle--which are not very attractive, but are a needed composter in your working pile of organic matter, as they are one of the best feeders the compost pile will have.
Gently dig down about 1 foot into your compost pile with a garden trowel or pitch fork. Look for crawling, squirming worm-like creatures that have a cream-white colored mid-section, rusty red-brown head and gray back end. Grubs will pupate or emerge from eggs at about a 6-inch depth into the soil.
Notice that the plump worm is 3/4 inch to 1 1/2 inches long and has a rounded, multi-segmented body that forms a C shape. They have a look similar to a peeled baby shrimp.
Move the white grub to see if it will flip on its back in an attempt to get away and burrow back down into the compost. This movement is particular to the Green Japanese Beetle grub. Note that the three pairs of legs are located in the top front of the segmented body of any type of grub. The top (and smallest) pair of legs jut out at the sides just under the head of the worm, allowing it to grab at the soil and burrow away. The rest of the body is legless.
Look at the head area of the grub and note that the larvae has a mouth. The grub feeds on the organic material that is decomposing in your compost bin.
Use a small magnifier to view the underside of the grub. The bottom rear area of the grub has an anal slit where fecal matter is excreted. You will also notice several hairs or spines near the slit. The slit and spines help determine the species of beetle the grub is, due to recognizable shape and patterns of the slit and spines. The fecal matter, or casting, is an addition to your compost.
Cover the grub worms up and let them continue working in your compost pile.