April 13, 2013
1 - Solarization can be used to pasteurize compost before use in the garden if you have insects or concerns about diseases in the compost.
Yard and Garden April 13, 2013
My compost is loaded with huge fat white grubs. This is the first year this has happened. What went wrong and what should I do to get rid of them?
Thanks, Sylvia O.
I cannot be certain, but these are probably the larvae of the ten-lined June beetle or the large, metallic green beetles that we often see flying around in the summer. They are usually not as much problem as the lawn grubs (regular June beetles), but can cause some problems. They will soon turn into pupae as they become the adult beetles. These pupae are segmented and I think they look like little armadillos. To get a better identification and determination of whether or not they can be a pest in the garden, you can take some of the larvae (alive) to your local NMSU County Extension Service office. The agent may be able to identify them, or can send them to NMSU to have an entomologist identify them and make recommendations.
Until then, there are several things you can do: As you harvest the compost to use in the garden, put it in a fairly thin layer (6 inches) and cover with a sheet of clear plastic in a sunny location. Let the sun shine through and heat up the compost for a few days. This is called solarization. If the temperature reaches 160 degrees in the compost, the larvae and most plant diseases and weed seeds will be killed. The compost can be safely used in the garden. This may not be necessary, but until you know for sure, it may be the safest thing to do.
You can try to heat the compost naturally by adding a source of nitrogen (manure, green grass clippings, kitchen wastes, etc.) to the compost pile. This should activate the composting organisms and generate heat. You will need to turn the compost so that all the grubs spend time in the hot center of the compost. This will not be as effective as solarizing, but it may help. The fact that the grubs are there indicates that the compost was not hot enough to kill them - it is never hot enough through the entire mass of the compost and the insects (not the pupae, however) can migrate away from the hottest regions.
Finally, there are some insecticides that may be useful. If the insecticide is labeled for safe use in the vegetable garden or flower garden (depending on where you will use the compost), it can be used in the compost as well. Results from your County Extension Service office will help determine which insecticide to use, if you want to use an insecticide.
Follow-up comments from the gardener: Your information is extremely helpful. We do have a lot of those fluorescent green beetles during the peach season but since we are organic farmers we hesitate to use any pesticides. Your suggestion of solarization is doing the trick! I have a wheel barrow load and I have solarized it for 2 days. The grubs are dead when I turn it. I still remove them and smash them, just to be sure.
Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: firstname.lastname@example.org, office: 505-865-7340, ext. 113.
For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms.
Please copy your County Extension Agent and indicate your county of residence when you submit your question!