Insect I.D.: Nov. 2000, Battling Pests, Biological Control

Soil-inhabiting pests have many predators, parasites and diseases that attack them. Many of these are naturally occurring organisms but most rarely act rapidly enough alone to adequately control pests. Because of this, biological agents are usually reared and released into the environment with the target pest. This process is called augmentation. In a few rare instances, bilogical control agents not normally present have been introduced into the turf habitat. These foreign introductions are most commonly used to control introduced pests.

PREDATORS AND PARASITES. Though naturally occurring predators and parasites are occasionally effective in suppressing turf insect pests, they usually have unreliable and unpredictable efficacy. In some cases, predators and parasites, such as spiders and wasps, cause more concern than the target pest.

The wasp, Scolia dubia, is a parasite of the green June beetle has been effective in lowering grub populations below damaging levels. Unfortunately, these large, often hairy wasps can cause great alarm in people afraid of being stung. Actually, these wasps rarely sting humans, and then only when severely provoked. Other parasitic wasps, Tiphia spp., parasitize masked chafers, May-June beetles and Japanese beetles, but most of these wasps rarely control their hosts on a regular basis.

PATHOGENS. Insect pathogens (diseases) have been the most promising and effective biological control agents. Bacteria and fungi are often easily reared or produced in artificial media. Many of these microbes also produce resistant spores that may be formulated and distributed. In the past, only a few pathogens were commercially available but recent production and formulation techniques are making more available.

BT FOR WHITE GRUBS. The bacterium, Bacillus thruingiensis (commonly called BT), is a common bacterium found in soils. When first discovered, some strains contained toxin granules that seemed to affect the gut lining of certain insects, mainly leaf feeding caterpillars such as sod webworms and armyworms. Since discovery of this original group of strains, over 10,000 different strains have been characterized.

The white grub active strain is BT variety japonensis, strain ‘buibui.’ Initial field tests have shown promising efficacy (70 to 90 percent control) against Japanese beetle and masked chafer grubs. Other grub species may be controlled, but to a lesser extent. At present, no commercial preparations of this strain are available.

BTs are used differently than milky diseases. BT bacteria are easily produced in large fermentation tanks, in vitro. Spores of BT are fairly susceptible to degradation from sunlight and other microbes may kill the spores. Therefore, this BT is applied using "inundative augmentation." In other words, large numbers of spores are applied to the area where white grubs are active, similar to broadcast application of an insecticide. The infection is rapid and maximum control is achieved in a few days to two weeks. Though some BT spores may survive until the next season, most simply decompose. Annual application is needed whenever the grub populations reappear.

MILKY DISEASE OF BTA. Black turfgrass ataenius (BTA) grubs are susceptible to infection by a specific srain of Paenibacillus popilliae. Occasionally, significant numbers of BTA larvae are infected with this naturally occurring strain. Infected larvae are opaque white and, though still alive, are not as active as healthy grubs and stopped feeding. The presence of infected grubs should be viewed as "good news" because dead larvae release millions of spores that remain to infect future generations of BTA.

The BTA-infective milky disease has caused some confusion. Examination of turf one to tow weeks after application of an insecticide for control of BTA larvae sometimes reveal considerable numbers of milky gubs. Apparently, the milky larvae survive the insecticide while the healthy grubs are killed. One might conclude that the insecticide failed and another application is warranted. In fact, this result is ideal since healthy grubs are killed and the surviving milky disease infected grubs will die of the disease and add to the spore residue in the soil. Our theory is that infected grubs stop feeding and therefore survive because they do not ingest the insecticide.

BT FOR WHITE GRUBS. The bacterium, Bacillus thruingiensis (commonly called BT), is a common bacterium found in soils. When first discovered, some strains contained toxin granules that seemed to affect the gut lining of certain insects, mainly leaf feeding caterpillars such as sod webworms and armyworms. Since discovery of this original group of strains, over 10,000 different strains have been characterized.

The white grub active strain is BT variety japonensis, strain ‘buibui.’ Initial field tests have shown promising efficacy (70 to 90 percent control) against Japanese beetle and masked chafer grubs. Other grub species may be controlled, but to a lesser extent. At present, no commercial preparations of this strain are available.

BTs are used differently than milky diseases. BT bacteria are easily produced in large fermentation tanks, in vitro. Spores of BT are fairly susceptible to degradation from sunlight and other microbes may kill the spores. Therefore, this BT is applied using "inundative augmentation." In other words, large numbers of spores are applied to the area where white grubs are active, similar to broadcast application of an insecticide. The infection is rapid and maximum control is achieved in a few days to two weeks. Though some BT spores may survive until the next season, most simply decompose. Annual application is needed whenever the grub populations reappear.

AMBER OR HONEY DISEASE OF WHITE GRUBS. This disease is caused by the bacterium, Serratia entomophilia. Infected grubs stop feeding and their body fluids become a honey-amber color. Affected insects become flaccid in a few week and soon decay. Though Serratia is found around the world (including the United States), the only commercial product is available in New Zealand where it is successfully used for management of the grass grub in sheep pastures.

WHITE FUNGUS DISEASE. The fungus, Beauveria bassiana, is often called the white fungus disease of insects because it covers infected insects with a snow white fungal mass. Infected insects become sluggish and eventually stop all activity. Within a few days or weeks the fungus that has been growing within the insect body sporulates by forming a dense cottony mass over the insect exterior. Many strains have been identified and almost all turfgrass-infesting insects are susceptible. Chinch bugs and billbugs are commonly infected and their populations severely reduced by this fungus.

Beauveria outbreaks often occur in periods of rainy, cool weather. Though white grubs and mole crickets are susceptible, their populations are rarely controlled adequately. A recent product, Naturalis-T™, has been marketed for management of mole crickets and chinch bugs.

Apparently, the key to obtaining maximum efficacy from Beauveria application is to keep the turf thatch and soil moist for a week to 10 days after application. Naturally occurring Beauveria is also most effective where the turf is kept moist. Application of fungicides destroys this fungus.

GREEN FUNGUS OF INSECTS. Metarhizium anisophiae forms an olive-green spore coating on infected insects. Infected insects become sluggish and stop all activity. The fungus grows within the insect body and soon coats the exterior surface with white mycelia. These mycelia sporulate, producing the greenish coloration.

Though several strains are being developed by foreign and United States companies for management of white grubs, no commercial products are yet available.

ENTOMOPATHOGENIC WORMS. These nematodes are specialized roundworms, which carry a bacterium lethal only to insects. Juvenile nematodes usually enter an insect through the mouth, anus or breathing pores, though some species may be able to penetrate through the insect exoskeleton. Once inside, the nematode regurgitates a bacterium. The bacteria multiply, generating a toxin that kills the insect and prevents other bacteria multiply, generating a toxin that kills the insect and prevents other bacteria from colonizing the cadaver. The nematodes feed on the bacteria, mature and reproduce inside the body of the dead insect before releasing a new generation to seek other hosts. These nematodes are not harmful to animals other than insects and they can not enter plant tissues.

Steinernema nematodes are commercially available under several trade names. S. carpocapsae is the most commonly produced species because of the ease of producing juveniles in large fermentation tanks. S. carpocapsae is most useful for control of cutworms, sod webworms, billbugs and fleas. However, the nematodes are very susceptible to desiccation, can not tolerate direct sunlight, and may be killed by certain insecticides or fungicides applied to turf.

S. feltiae and S. Glaseri are also marketed for surface insect and grub control. Steinernematid nematodes, in general, have not performed well for control of grubs.

S. riobravae and S. scapterisci are species registered for control of mole crickets and properly made applications have produced satisfactory control.

Heterorhabditis bacteriophora nematodes are commercially available from smaller suppliers. Recently, larger scale production of this nematode under the name of Cruiser™ has been accomplished. Heterorhabditid nematodes have generally been the best performing species for control of grubs.

Best efficacy has been achieved when the nematodes are applied in the late afternoon or evening (to avoid exposure to direct sun light), to moistened thatch and soil, followed with immediate watering-in.

BIOLOGICAL CONTROLS AND THE TARGET PRINCIPLE. The essence of the Target Principle as described earlier in this chapter is that materials directed at controlling damage from soil-inhabiting insects must reach their primary feeding and/or activity zone to be effective. Current methods of applying biological control agents for pests such as grubs, usually involve broadcast or spot applications to the turf surface followed by irrigation or rain to move the agent to the activity-feeding zone of the target insect. A major reason for the limited (at best) success of these agents is that they simply do not reach the target in sufficient quantities to be effective. The physical condition and characteristics of the medium above the Target presents a major impediment to their downward mobility. Until methods and equipment are devised that deliver these agents directly into the target zone, without unacceptable damage to the turf surface, biological controls will continue to have limited success and acceptance.