Not All Grubs Are Alike
"Grub" has many meanings. Grub can be a verb that means to dig; for example, pigs grub for food. Grub can be a noun meaning a hearty food or a beetle's larval stage. You’ll have trail food for dinner if you've ridden up to the chuckwagon for grub. However, if you find divots on your lawn, perhaps a hungry skunk has invited himself for a meal of grubs in your yard.
The beautiful metallic green and bronze Japanese beetle (Popillia japonica) is common throughout the eastern United States. These adults chew holes in the leaves of our favorite roses, grapes, cherries, and more! Suddenly they seem to be everywhere. Where did they come from so quickly at the end of June? And what do they have to do with grubs?
Let's take a step back to biology class. Beetles are the most significant order of insects, with over 400,000 species! Within this order are the Scrabaebidae, the scarab beetles. In ancient Egypt, they represented the cycle of life and death and renewal and rebirth. Scarabs, like other beetles, have complete metamorphosis with four life stages: egg, larva, pupa, and adult. Adult scarab beetles lay their eggs on the soil surface. The eggs hatch into tiny white grubs with soft bodies that curl into a C-shape in about two weeks. They have six legs and chewing mouthparts to feed on turfgrass roots and other plants. As they feed, they grow larger and molt their soft exoskeleton. After feeding, they become a non-feeding pupal stage (rather like a mummy). From mid-June to late June in Pennsylvania, adult Japanese beetles emerge from the pupae. These adult beetles began life as an egg in early July of the previous year. They spent most of their lives as larvae feeding on plant roots.
So, the white grubs that eat the roots of your turfgrasses are juvenile Japanese beetles, correct? Well, yes, but maybe not! There are many species of scarabs. Several are pests as adults; all are pests when grubs. The adult beetles are very different in appearance, size, and color. It’s easy to tell the adults apart! But, all white grubs begin life as small grubs, and all are white. Some grubs become surprisingly large (such as the green June beetle)! Knowing what kind of grubs are damaging your lawn is essential to manage them correctly.
Since so many different kinds of scarabs cause trouble to our plants, how do you identify them in the larval (grub) stage? Using a powerful hand lens or microscope, examine the pattern of hairs on the end of the grub's abdomen. Yes, look at their little hairy butts! The raster pattern of hair and the shape of the anal slit will identify which scarab beetle larvae are grubbing in your yard. Ten to fifteen grubs is an acceptable sample, but more is better. Most of the white grubs in my area are not Japanese beetle grubs but are a different, introduced species, the oriental beetle (Anomala orientalis). The oriental beetle adult is tan with brown markings and smaller than a Japanese beetle. I often see it feeding on flower petals in June. It was discovered in Connecticut about a hundred years ago and is slowly spreading south and west. Another scarab that may cause trouble as a grub is the Asiatic garden beetle (Maladera castanea). Common names are very confusing!
While the damage is done to roots by small, hungry white grubs in autumn (September and October), usually, the resident doesn't notice the damage until spring (April and May). During the spring, very little damage is done by the grubs. At this time, the grubs are large and away from the surface. It is challenging to manage any kind of grubs in the spring. Read on for possibilities that will help you, depending on the species of grubs that you want to control.
Pheromone and floral-scent traps attract adult beetles. Researchers created these to monitor the introduced Japanese beetle's spread throughout the United States. The traps were devised as a monitoring tool, not a management tool. They work well, definitely too well. These traps attract more beetles to an area than they catch! The tongue-in-cheek instruction that Master Gardeners give to clients is, "Put the traps far from the crop you want to protect. Place the traps in the neighbor's yard, a neighbor you don't like!" Most pheromone traps are designed for only Japanese beetles.
A bacterial milky disease (Paenibacillus popilliae) kills Japanese beetle grubs as they move within the soil. This product is available in two commercial forms. Each has specific instructions for application to the surface of the lawn or field. It may take three to five years for the grub disease to become established. It is effective only on Japanese beetle grubs, not any other species.
Microscopic nematodes that are parasitic on grubs are available to suppress white grub species. Steinernema spp. and Heterorhabditis spp. nematodes are available. These are living organisms and must be handled appropriately before and after application to be effective. Moist soil is required for these roundworms to survive. Steinernema nematodes stay near the surface and ambush grubs. Heterorhabditis nematodes are active hunters that seek grubs. The infested grubs produce successive generations of nematodes to continue the management of this technique.
Bacillus thuringiensis galleriae (Btg) is a naturally occurring bacterial insecticide. It manages over ten species of grubs. It is related to Bt israelensis, which works on mosquito larvae, and Bt kurstaki, which works on caterpillars.
Many chemical insecticides are designed to manage lawn and soil insects. Some work well on surface insects; others control both surface and soil insects. Remember that grubs live in the soil. The pyrethroid group of insecticides includes bifenthrin, cyfluthrin, deltamethrin, gamma-cyhalothrin, lambda-cyhalothrin, or permethrin. This insecticide group controls only surface insects. As the chemical moves into the soil, it binds with organic matter and becomes ineffective against grubs. It is essential to read the list of managed insects on the package—specifically, note which insects are listed and whether they are adults or grubs within the soil. For example, trichlorfon controls white grubs (larvae of chafers and Japanese beetles). However, no other scarab beetle grubs are listed because it is not effective on them! When used according to directions, a different chemical, chlorantraniliprole, is effective on all grubs and fall armyworms. A systemic insecticide, imidacloprid, is effective on grubs. However, this insecticide travels up to any flowers, for example., clover, dandelions, and others blooming in the lawn. The nectar and pollen will be affected, and desirable pollinators will be impacted.
The concept of integrated pest management (IPM) uses the beetle's and grubs' likes and dislikes to manage these pests. Correct identification of the pest is the first step of IPM. Cultural control is the next step. Your local Penn State Extension Master Gardeners will assist with your grub, the insect, not the meal.
