There’s a new insect pest to have on your radar in Wisconsin—the Asiatic garden beetle (Maladera formosae; formerly Maladera castanea). This species can feed on and damage a wide range of plants including, vegetable crops, field crops, fruit crops, turfgrass, and ornamental flowers, trees, and shrubs in nursery and landscape settings.

Origins and History: The non-native Asiatic garden beetle is originally from parts of eastern Asia and was first detected in the US (New Jersey) in 1921. It can now be found across much of the eastern US. Over the last two decades, AGB has become more common in the Midwest with crop damage being reported in Indiana and southern Michigan. It is also established in parts of northern Illinois. Asiatic garden beetles were first collected in Wisconsin in July 2021 in a residential yard in Dane County (Middleton). Small numbers of adults continue to be spotted at that site, although no feeding or plant damage has been observed to date. In late July 2023, Asiatic garden beetles were collected from a home garden in eastern Green Lake County; this is also the first confirmed report of plant damage in the state.

Appearance: Asiatic garden beetles belong to the Scarab family and the larvae (white grubs) resemble other species in the group (Japanese beetles, May/June beetle, etc.). Larvae have pale, C-shaped bodies with three pairs of jointed legs and a brownish-orange head capsule and chewing mouthparts. They have a pale, bulbous structure at the base of their mouthparts which aids in diagnosis (no other white grubs in the Midwest have that feature). Adults AGBs are approximately 3/8 inch long, brownish, and resemble small May/June beetles; their elytra (wing covers) are also slightly iridescent. Closely-related members of the native genus Serica can look almost identical in appearance and are best separated under the microscope. The structure of the hind legs and the number of antennal segments are important features used in taxonomic keys to separate AGB from our native Serica species, which are not considered pests.

Life cycle and Biology: The Asiatic garden beetle has a single generation each year. Adults have primarily been spotted in July in Wisconsin, although some specimens have been collected as late as September. Adults are nocturnal and feed almost exclusively after dark. If disturbed, they tend to tumble to the ground and hide. Adult AGBs are capable fliers and can come to lights in large numbers; blacklight traps can be a useful monitoring tool. In addition, adult flight activity is strongly associated with warm nighttime temperatures (70+˚ F). After mating, adult females lay eggs in soil. Reports from nearby states indicate that larvae may be more common in sandy soil compared to loamy areas.

Damage: Both the larvae and adults can feed on a wide range of plants. Adult AGBs tend to pack less of a punch than Japanese beetles, but can chew irregular notches out of leaves. The larvae can be more of an issue in crop fields when they damage roots and other below-ground structures (tubers, etc.). An abundance of AGB grubs can lead to stunted plant growth in spring and below-ground wounds could serve as a potential entry point for pathogens. AGBs have been reported to damage 100+ different types of plants, including: beans and peas, cole crops, beets, potatoes, and carrots, corn and soybeans, Solanaceous crops, fruit trees, strawberries, caneberries, common weeds, and many other plants.

If you suspect that you’ve found Asiatic garden beetles, please collect a sample and submit specimens to the UW Insect Diagnostic Lab to confirm, as we’re tracking this pest on a county-by-county basis. Sample submission instructions for the UW Insect Diagnostic Lab can be found here: https://insectlab.russell.wisc.edu.

This article was posted in Features and tagged PJ Liesch.