San Diego composters and gardeners have long expressed an interest in the wiggly, white grubs living in their bins and garden beds. While grub species vary depending on your location, residents of the western/southwestern United States and Mexico are likely referring to one specific insect native to our region.
WHAT ARE THEY?
Digging around your backyard compost bin, you may notice a plentiful population of white, soft-bodied creatures with a somewhat extraterrestrial quality. No, this isn’t first contact; these are the larvae of the Figeater beetle (Cotinis mutabilis).These larvae can grow up to 2 inches long, and are commonly spotted at rest, rolled into a tight “C” posture. Figeater beetle larvae have been aptly nicknamed “crawly backs” for their interesting method of mobility; these larvae propel themselves by flipping upside down and using the stiff hairs on their backs to create traction, rendering their six tiny legs essentially afunctional.
WHAT ARE THEY DOING IN MY COMPOST BIN?
The compost bin makes a hospitable nursery for figeater beetle larvae, which dine almost exclusively on organic detritus. These larvae are highly beneficial residents in the bin, helping to accelerate decomposition and diversifying the nutrient composition of the pile. If you dig deeply enough, you’ll spot a network of tiny tunnels that attest to their hard work enriching your compost.
ARE THEY SAFE TO ADD TO MY GARDEN?
Despite being an invaluable asset inside of the compost bin, figeater beetle larvae have the potential to become a garden nuisance if mixed into a growing medium in large quantities. A larval population surge wouldn’t be expected to do physical damage to root and plant structures as much as simply over-compete with your plants for nutrients. You can prevent transplanting a large population of crawly backs by screening your finished compost and tossing the larvae back into the compost bin to convert nutrients in peace (you can even leave a few out in a bucket as snack for local birds.) Don’t fret if you see a few grubs while digging around your beds; it’s perfectly natural to find these larvae hanging out in the soil, and their presence is nothing to lose sleep over. Mature Figeater beetles do dine on nectar, pollen, flower petals, and fruits but are unlikely to do significant damage to crops unless found in massive populations. (Expect squirrels and birds to consume more fruit than Figeater beetles).
WHAT ABOUT ALL THOSE HORTICULTURAL HORROR STORIES?
Mature Figeater beetles are often confused with two other species that are known to cause significant root and foliar damage to plants: the Green June beetle (Cotinis nitida) and the Japanese beetle (Popillia japonica). Both larval and mature Green June beetles and Japanese beetles are known to infest fields, killing off grasses, grains, and fruits by stealing soil nutrients and excessively consuming roots and foliage. While they are all members of the scarab beetle family, Figeater larvae and beetles are far less destructive than their cousins residing in the eastern/southeastern United States. In the unlikely event that you do experience an infestation of Figeater larvae in your garden bed, there are plenty of natural solutions that will control the population, including beneficial (parasitic) nematodes, homemade remedies, and manual removal.
So worry not, west coast composters! You get to enjoy the benefits of catalyzed decomposition and nutrient enrichment that Figeater larvae provide without the stress of managing mature beetle populations.