Knowledgebase
Grub control #877003
Asked July 12, 2024, 2:41 PM EDT
Hello, I have some raised bed vegetable garden beds in Highlands ranch. I've had Japanese beetles for the past few years and have attempted to manfully control with hand picking. Last year I used traps which Im afraid just brought in more. This summer, I've found hundreds of white/grey grubs in those beds...I assume they are Japanese beetle grubs. Being that its a vegetable garden I'd like to stay away from pesticides if possible. Ive been hand picking the grubs every few days and seen a few adult Japanese beetles, but nothing terrible yet. I've covered the beds with insect netting. Would the grub be gone or any other nematode product be effective against a wide array of grubs? Im guessing I need to treat the lawn as well? Thanks in advance for your thoughts
Douglas County Colorado
Expert Response
Hi John,
Sorry to hear about your Japanese beetle problem--yes, they are a real pain!
It's unlikely that the grubs you're finding in your vegetable garden beds are Japanese Beetles; they feed on living plant roots and prefer turfgrass (so females would be unlikely to lay eggs in the garden if a lawn is nearby). More likely are the larvae of bumbleflower beetles, which feed on decaying organic matter and are commonly found in both compost piles and highly amended soils. If you wanted to be sure, you could check the pattern of hairs on the grubs' hind end, known as the rastral pattern. Japanese beetles have a noticeable "v" shaped rastral pattern (see the image in this fact sheet: https://extension.colostate.edu/topic-areas/insects/japanese-beetle-5-601/).
Beneficial nematodes Heterorhabditis bacteriophora and Steinernema feltiae are commercially available and can be effective against all sorts of beetle grubs, though, and could get rid of those in your vegetable gardens if you were to discover that they were damaging plant roots.
You would only need to treat the lawn if you noticed lawn damage; treating for Japanese beetle grubs will not affect the number of adults you will find, as they are highly mobile and will fly in from quite far away. And you're right, the traps attract more beetles than they catch!
Exclusion is a great option to protect your plants. Hand-picking both the beetles as well as any damaged flowers or foliage will help make the garden less attractive; the beetles are attracted to damaged foliage especially.
Another organic option for control is the bacterium Bt variety galleriae, sold in products like BeetleJUS and BeetleGONE. These can be sprayed on most plants up to the day of harvest and are not a hazard to bees.
Be sure to read and follow the label instructions for any pesticide, organic or otherwise.
I hope this is helpful!
Cordially,
John
Hi John,
Usually, extra water would mask the symptoms of JB larvae.
For the lawn, Milky Spore is not a useful control (it might establish, but you won't notice any measurable change in grub numbers). The nematodes or a bacterial product (Bt var galleriae, sold as grubGONE!, for example) would be better investments.
I hope this is helpful.
Cordially,
John
