Insect Control Depends on Damage Done

Q. I grow a lot of different plants in nursery containers in my yard. This past spring I found  three white grubs in the soil in one 5-gallon container! I also found them in the soil in my yard when planting new roses and trees. I’ve been manually picking them out and tossing them on the driveway for the birds to eat when I see them. Are they a threat to my flowers and veggies? Should I take any action against them?

A. Finding large white grubs in the landscape where soil has been improved is normal during the spring months. However, finding a lot of white grubs in landscape soils is not a good thing because they can eat the smaller young roots of plants as well. Personally, I wouldn’t be overly worried if I found one white grub in a 5-gallon nursery container full of soil but three is excessive. I would take some action.

            This is the immature grub of a scarab beetle, sometimes called a June beetle. When temperatures begin to warm they pupate in the soil and emerge in summer as a flying adult seeking the opposite sex and a place to lay its eggs if it’s female. Your landscape sounds like it’s a playground for these guys!

Green metallic june beetles devouring Kadota fig

            Probably one of the most frequent “June beetles” we see flying in the summer months is the metallic green June beetle. When flying out and about they love to eat white Kadota figs and just about any soft fruit that’s ripe! But we also see during summer months the brown June beetles as well which are another type of scarab beetle.

Ten lined June beetle in the orchard another chafer or scarab beetle

            Adult female beetles lay eggs in mid-summer. The female picks spots to lay her eggs where there’s plenty of food for her young to eat. She hones-in on soils that have been amended with compost since these young white grubs favor ripe or decaying organic plants mixed in the soil. Young tender roots of plants are mingled with decaying soil organics and they’ll feast on both. That’s the problem.

Another June beetle picture sent to me by a reader

            Traditional pesticides like Sevin insecticide applied to the area as a soil drench will easily kill these grubs if you lean in that direction. Sifting through the soil when you’re planting and handpicking them like you’re doing will get rid of most of them if you are organically inclined. Using Milky Spore bacterium for biocontrol only works if these white grubs are related to Japanese beetles. Most of our white grubs are not.

            A preventive measure for future years is mixing beneficial nematodes into the soil that pursue various types of white grubs and not just the Japanese beetle. Your local garden center may carry beneficial nematodes, but they should be refrigerated when buying them. The best time to apply them is usually around August after egg laying has finished and the young grubs are starting to gain some size.

            Beneficial nematodes should be kept in the dark and in the refrigerator until they are ready to be mixed in the soil. They will have directions for their use on their label.