These control strategies can help mitigate damage to turfgrass
The European chafer (Amphimallon majale) is a beetle that feeds on turfgrass roots causing substantial damage. In more recent years, this invasive insect has been confirmed in the Pacific Northwest and British Columbia. Populations have been observed as early as 2015 in Portland, Oregon and Seattle and Tacoma Washington areas. Their numbers have been increasing ever since.
Identification and life cycle
The adult European chafers can be found above ground in late June and early July. This thick-bodied beetle is about ½-inch in length, larger than a Japanese beetle (Popillia japonica), but smaller than a green June beetle (Cotinis nitida). This beetle has a light-brown or tan colored head, thorax, abdomen and wing covers (Image 1). The insect’s eyes are dark-brown to black, and the wing covers have grooves running the length of the insect. The abdomen protrudes slightly beyond the wing covers at the rear end.
The adult stage of this insect is short-lived, at one to two weeks. During this time, the adults find partners, mate, and then lay eggs before dying. Each female European chafer will lay 20–40 eggs, one at a time, in the top 2–4 inches of moderately dry soil. The eggs are one-tenth of an inch long, thick and white to a dull gray in appearance. Larva hatch from these eggs about two weeks after being laid.
The larval stage of this insect lives underground from late July to the following May (11 months). This larva is a large, C-shaped grub. Chafer grubs have a white body, light-brown head, three pairs of light-brown true legs, and a dark section at the end of the abdomen (Image 2). The European chafer raster pattern, which is used to distinguish between various species of white grubs (European chafer, Japanese beetle, and June beetle), includes a Y-shaped anal slit and two rows of parallel spines running toward the front of the grub.
These grubs reach maximum length of one inch in the fall and cause the largest amount of damage to the rooting systems of grass plants in the subsequent spring. In May and early June, the chafer larvae form pupae in preparation for the transition to the adult stage of their life cycle. The pupae are just over ½-inch in length and stout with a light-brown appearance. In late June and early July, adult chafers emerge from the pupae and leave the soil to complete the life cycle.
Symptoms of chafer grub activity include localized patches of drought-stressed turf appearing in the spring and early summer months. These patches of turf easily peel up from the soil because the grubs have eaten the root system. At this stage, insecticide applications will not make a difference because the root damage has already been done. These areas often become desiccated during the summer months, and will require replacement with seed or sod. Skunks, raccoons and birds will often forage for the grubs in infested areas, damaging the turfgrass in the fall and following spring (Image 3).
Scouting for European chafer grubs should be done in the fall (September and October). To scout for these grubs, dig up a 1-foot long by 1-foot wide by 2-inch deep section of sod with a square end shovel. Flip the sod layer over and inspect the soil for grubs (Image 4). European chafer prefer dry, sunny turf areas with little irrigation. Sloped turfgrass areas prone to surface runoff will likely be drier than flat turf area, and consequently could have higher rates of chafer infestation. Animals forging for grubs in the fall and spring are also a good indicator of infested turfgrass.
The typical action threshold (pest population) that will cause damage if untreated is 5–10 grubs per square foot for low maintenance turf, and 15–20 grubs per square foot in high maintenance turf. Considering this, adequate implementation of the primary cultural practices (mowing, fertilization and irrigation) is an important factor because this will increase resistance to damage.
High-maintenance lawns in the Pacific Northwest should be mowed at least once a week to a height of 2–3 inches. To improve nutrient levels, grass clippings should be returned using a mulching mower. High-maintenance lawns should be fertilized twice in the spring and twice in the fall at 1 lb. N per 1,000 square feet per application, totaling 4 lbs. N per 1,000 square feet annually.
Research has shown that frequent irrigation during late June and July can substantially mitigate pest populations. It is speculated that the adult chafers prefer to lay their eggs in dry soil, rather than moist soil. Frequent irrigation also improves turfgrass tolerance to root-feeding pests, such as the European chafer.
Considering this, irrigation events of ¼-inch deep, four times a week, totaling 1-inch per week are suggested for high maintenance lawns in the Pacific Northwest. An August and September application of parasitic nematodes (Heterorhabditis bacteriophora) is suggested, however data on the effectiveness of this control method is minimal. Similar to insecticides, biological control agents typically need to be applied annually when managing insect populations.
Before considering insecticides, always evaluate chafer grub populations. If the action threshold of 5–10 grubs per square foot for low maintenance turf, and 15–20 grubs per square foot in high maintenance turf is not reached, an insecticide application will not provide any benefit. If insecticides are going to be applied for control of this insect, the best results will be observed when treatments are made while the larvae are young, also known as early instar, grubs in the fall.
Spring scouting and insecticide applications will likely not prevent drought-related turfgrass damage in the summer months because the root system has already been compromised. In the spring months, the fully developed, or late instar grubs, are also resistant to some insecticides.
Effective preventative insecticides with relatively low animal (mammals and fish) toxicity levels include chlorantraniliprole, thiamethoxam, clothianidin, and imidacloprid. Chlorantraniliprole is the only insecticide listed that has a low bee toxicity. To protect pollinators, the remaining products should not be applied to turfgrass adjacent to blooming plants. These preventative products should be applied early in the chafers’ life cycle in July and August. Late applications of these products will not provide effective control.
Effective curative insecticides include thiamethoxam, trichlorfon , and carbaryl, but all have relatively high animal toxicity levels to mammals, fish and bees. These curative products are highly toxic to bees and should not be applied to turfgrass adjacent to flowering plants. Curative insecticides should be applied in the fall in September and October. Spring application of any insecticide will provide limited control.
At this time, the larva are at their final instar, making them more resistant to the insecticides. The larva have also been feeding on the turfgrass roots since the fall, causing a significant amount of damage going into the spring months.
European chafer populations are growing in the Pacific Northwest, and this insect is particularly damaging to turfgrass areas that receive minimal irrigation levels. Regular irrigation during the summer months, combined with frequent mowing and fertilization, will increase the action threshold for European chafer to 15–20 grubs per square feet.
If these populations are observed during scouting, preventative insecticides can be made in July and August, while curative insecticides can be applied in September and October. Curative insecticides that effectively control European chafers are more toxic to animals and pollinators than the preventative insecticides, so curative applications should be used as a last resort. Spring insecticide applications will have minimal effects because the damage has likely already been done and should be avoided.
Alec Kowalewski, associate professor and turfgrass specialist at Oregon State University (OSU), can be reached at firstname.lastname@example.org. Emily Braithwaite is a faculty research assistant at OSU. Chas Schmidt, Ph.D. is a turfgrass research assistant.