A reader recently sent in an extreme closeup photo of a scary-looking grub. It’s hard to make a positive identification without seeing the grub itself, but it’s either the grub of the hoplia beetle or the masked chafer beetle. In this case, the best clue is to find out where the grub was found. If it was discovered under or near the roots of lawn grasses, it’s probably a masked chafer beetle grub; if it was found just under the soil surface in an undisturbed, uncultivated area in the reader’s garden or in a nearby vacant lot or fallow field, it’s probably the grub of the hoplia beetle. Both look like white or grayish C-shaped fat worms or caterpillars with legs, brown or reddish heads, and are about an inch long. These grubs overwinter as mature larvae in our gardens – hatching out in spring as beetles.
Hoplia beetles are a half-inch long, silvery gray, and chew round holes in the petals of white or light-colored flowers (especially roses) in April and May. The larvae feed on decaying vegetation, but not on woody plant roots, from fall to early spring when they hatch.
Masked chafer grubs feed on lawn grass roots from August to June, killing the grasses, and causing irregular brown patches that resemble drought stress. In mid-summer when temperatures are high, the adult beetles hide in the soil and fly at night looking for mates.
Control of these pest insects in winter includes cultivating the top few inches of soil with a wiggle hoe to uncover the grubs and kill them. (Cultivation is also effective in uncovering overwintering cutworms – a caterpillar). Applications of beneficial nematodes in fall to the soil or lawns where the grubs have been found is also very effective. Applications of insecticides are not usually necessary. In cases of severe infestations and damage, imidacloprid can be applied through a drip system.
Beetle grubs or larvae are some of many pest insects that overwinter in our gardens either as adults or larvae or in the egg stage. Cold winter temperatures kill some pest insects and their eggs; our efforts to monitor for their presence in our gardens during the winter months and to remove or clean out hiding and breeding places can significantly reduce populations.
Stinkbugs including the Bagrada bug – a recently arrived pest – and the leaf-footed bug, squash bugs, whiteflies, aphids, grasshoppers and cutworms (the caterpillar stage of the cutworm moth) all overwinter in various stages of development in our gardens. We usually only recognize their existence when the damage by pest insect populations increases to levels that cannot be easily controlled by natural means such as beneficial insects. The Fresno County Master Gardeners website, www.ucanr.edu/sites/mgfresno, and the Master Gardeners working their hotline, 559-241-7534, 9 a.m.-noon Monday-Friday, can help identify the pest insects hiding in your winter garden. The University of California Integrated Pest Management website, www.ipm, ucanr.edu, has great photos, detailed descriptions of the insects, their damage, their preferred foods and hiding spots and recommended control methods.
Send Elinor Teague plant questions at firstname.lastname@example.org or email@example.com (“plants” in the subject line).