Maladera castanea (Arrow).
[Phylum Arthropoda: Class Insecta: Order Coleoptera: Family Scarabaeidae]
Asiatic garden beetle (AGB) was introduced from Japan in the 1920's. It is most common in northeastern United States from New England to Ohio and down into South Carolina.
Larvae occasionally attack turf but seem to prefer a variety of roots from weeds, flowers and vegetables. The adults feed on over 100 species of plants, preferring flowers of asters, dahlias, mums, roses and the leaves of a variety of trees and vegetables.
The grubs cause typical damage to turf, wilting and irregular patches of dead turf. Heavy infestations are uncommon but have been known to occur. Generally, the deeper feeding habit of asiatic garden beetle grubs results in less damage to turf than from other species. With adequate rainfall or irrigation, populations of 20 per ft² can be tolerated. Grubs prefer roots of other plants, therefore, they may be clustered around weedy areas especially near orange hawkweed. Grubs may also be concentrated next to flower beds where plants preferred by the adults are located. The adults feed at night, stripping foliage off of plants leaving a ragged appearance. They do not skeletonize like Japanese beetles. Flowers often have the petals eaten off.
Description of Stages:
The stages are typical of a scarab having a single generation per year. Since this beetle is rather small and larval stages are likewise small.
The eggs are laid in clusters of 3 to 15 loosely held together by a gelatinous material. The individual eggs are oval and about 3/64 inch (1.0 mm) long. After absorbing water the eggs become spherical.
Newly hatched larvae are about 1/16 inch (1.4 mm) long and have light brown head capsules. Full grown larvae are 3/8 to 1/2 inch (15 to 18 mm) long when stretched out. The grubs are commonly identified by the enlarged, light colored, maxillary palps (appendages just behind the mandibles) which appear to be in constant motion. AGB grubs can be identified by the longitudinal anal slit and transverse curved row of brown spines making up the raster.
The pupae rest in the last larval skin and are about 5/16 to 3/8 inch (8 to 10 mm) long. At first they are white and gradually turn tan.
The adults are 5/16 to 3/8 inch (7 to 10 mm) long and broadly wedge shaped. They are a chestnut brown and often have a slight iridescent, velvety sheen. The abdomen protrudes slightly from under the wing covers and the undersurface of the thorax has an irregular covering of short yellow hairs. The hind legs are distinctly larger and broader than the others.
Life Cycle and Habits:
Adult beetles may be active from late June to the end of October, but most of the adults are found from mid-July to mid-August. The adults emerge at night and fly actively when temperatures are above 70 F. When the temperature drops below 70 F, adults tend to walk up the plants or grasses to feed rather than fly. The adults are strongly attracted to lights. During the day, beetles hide in the soil around favored food plants. After feeding several nights the females begin laying eggs in small clusters. The females tend to search out turf and pastures for egg laying and generally deposit an average of 60 eggs one to two inches deep in the soil. Eggs are laid over several weeks and normally hatch in 10 days during summer temperatures.
Young larvae dig to the soil surface where they feed on roots and decomposing organic material. Most first instar larvae are found in August and early September. Second instars are found in September and many do not reach third instar until the following spring. About half the population overwinter as second instars and the remainder as partially developed third instars. As cool October temperatures arrive, the larvae burrow down 8 to 17 inches to pass the winter.
The larvae return to the soil surface in the spring and all seem to mature by mid-June at which time they pupate 1.5 to 4 inches in the soil in compacted earthen cells. The pupal stage is relatively short, lasting 8 to 15 days. The adult remains in the old pupal skin, changing from white to the mature chestnut brown, for a few days before digging to the surface.
Cultural Control - Habitat Modification -
Since the eggs require moisture for development, restricting irrigation in late June into early August may significantly reduce survival.
Natural Control - Milky Diseases -
Milky diseases caused by strains of Bacillus popilliae are known but commercial preparations do not contain Asiatic garden beetle-specific strains.
Biological Control - Entomophagous Nematodes -
The insect parasitic nematode, Steinernema glaseri Steiner, was used before 1940 and had considerable promise but this agent was not developed further because of problems of rearing and expense. This nematode may be economically available in the future. Commercially available products containing strains of S. carpocapsae have been marginally effective. Preparations containing Heterorhabditis spp. seem to be the most effective of the currently available nematodes. Nematodes should be applied when white grubs are in the second instar. Irrigation before and after nematode application with 1/4 inch of water minimum increases efficacy.
Chemical Control - Insecticides -
The adults can be monitored using 15 Watt black light traps. Trap counts can be plotted on a graph of numbers versus date. When the trap capture has decreased for ten days, mark the date of peak activity. The grubs will be most susceptible to controls about six weeks after this peak. Grub infestations should be evaluated as early as possible by monitoring, accurate identification and mapping of infested areas. The standard golf course cup cutter is a convenient tool to survey for infestations which are usually concentrated around shrub and flower beds. If care is exercised, sampling can be done with minimal damage to the turf. Once removed, samples can be examined carefully on the spot. Each soil and turf core is placed back in the hole made by the sampler. The sample does not have to be completely torn apart to determine the number of grubs present. If the soil is dry, it is advisable to add water to the sample hole before replacing the sample to improve the turf survival. The standard cup cutter is 4-1/4 inch in diameter. Therefore, to convert the number of grubs found per sample to the number of grubs per square foot, multiply the average per sample by a factor of 10.15.
Golf Course - AGBs rarely attack the turf of roughs. They are most numerous near flower beds and other landscaped sites that have small trees or shrubs. A few cup changer samples taken from the turf surrounding these areas should reveal any infestation.
Lawns, Grounds and Athletic Fields - Sufficient samples should be taken to determine the location and severity of grub infestation (i.e., mapping). Samples taken on a 10 foot grid pattern (larger distances for very large lawns) is often recommended. For detecting AGBs, concentrate samples along flower beds and landscape islands containing trees and shrubs. While using a cup changer works well, using a knife or spade to cut V-shaped cuts in the turf which are pulled back, examined for grubs and replaced, also works well. Though less precise, this method will provide a general map of infested areas.
Hand Lens. The hand lens is a most useful tool for identifying insects in the turfgrass environment. Use of a hand lens also projects a professional image to those nearby. Generally, lenses of 10X are adequate for most purposes, including identification of white grub species.
Many books and extension publications on grub management often mention "population thresholds." Thresholds are the numbers of pest insects present in a given area (usually per square foot) that warrant control. The principle behind using thresholds is to reinforce the IPM principle that application of a pesticide is not always warranted simply if pests are present. Pests must be present at populations high enough to eventually cause damage.
The generally accepted level for AGB is 10 to 15 grubs/ft², before control is warranted. However, well maintained turfgrass with regular irrigation and fertilization can "tolerate" much higher grub populations. On the other hand, moles, skunks and racoons often find less than 6 to 8 grubs per square foot sufficient to dig up the turf in search of them.
Thresholds must be adjusted for each turf situation. For golf courses, damage in roughs is more tolerable than damage in fairways, and damage in fairways is more tolerable than damage on tees and greens. Likewise, some course or home owners demand high turf standards while others may tolerate some periodic, localized damage.
As with most soil-inhabiting insects, insecticides are most effective when small stage grubs are the target. For AGBs, the small first instar grubs are present July through early August, unless a severe drought has occurred. In this case, the adults may delay egg laying and sampling should be performed about four weeks after normal soil moistures return. Applying an insecticide at this time, followed by sufficient irrigation to move the insecticide to the grubs usually yields satisfactory results.
First instar grubs molt into second instars that feed from mid-August through September. These middle stage larvae are also very susceptible to control, but if not controlled, turf damage may begin to appear by mid-September. From late September to October, AGB grubs reach the third instar stage and are 30 to 50 times the body weight of the newly hatched, first instars. By this time, significant damage may be caused by skunks and racoons who dig up infested turf to feed on the grubs.
Insecticide applications made in late September and October yield poor control of third instar grubs. At this time, insecticides known to have rapid action and are least affected by thatch binding are effective. In late April and May, AGB grubs return to the upper soil level to briefly feed. Though grub damage may be evident at this time, skunk and racoon damage is usually the major problem. If necessary, insecticides with rapid action may be used.
The recent labeling of long residual (>120 days) chloronicotinyl and diacylhydrazine insecticides has made it possible to obtain grub control with a single application in May, June, July or August. Depending upon the time of application, some of these new insecticides simultaneously provide control of a broad range of insect pests other than grubs. Unlike carbamate or organophosphate insecticides, these new materials have low inpact on beneficial organisms that live in the turf. Both liquid and granular formulations are available. Some require posttreatment irrigation, others do not. Read and follow label instructions for optimum results.
Preventive treatments are most warranted where the risk of grub infestation is greatest. Recent research has shown that turf areas that have experienced a damaging grub infestation are more likely to experience a reoccurring damaging infestation. Likewise, certain golf course areas and lawns in neighborhoods often have reoccurring infestations. These are likely candidates for preventive treatments.
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Asiatic garden beetle adult.
Asiatic garden beetle mature grub. Note white enlarged palp base under head.
Drawing of AGB raster.
Photo of AGB raster.
Asiatic garden beetles mating.