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Biological management of turf insects

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Paenibacillus popilliae


Milky disease (sometimes called “milky spore”) is caused by a bacterium, Paenibacillus popilliae, which occurs naturally in soil and affects the digestive system of Japanese beetle grubs. According to tests conducted in New Jersey, there is no evidence that milky disease is effective against other species of grubs. It is relatively nontoxic to people and other “non-target” organisms.

Milky disease is somewhat inconsistent in the Northeast. More turf managers have reported success with milky spore in the sandy soils of southeastern Massachusetts than in other areas. In any case, the milky disease organism may take several months to become effective; in some cases it may remain effective for three to five years. The discovery of “milky looking” grubs while monitoring may indicate that a milky spore application has taken hold. However, sporadic natural infestations of milky disease do occur, so the mere presence of milky grubs does not necessarily indicate that an application will be effective or consistent. Studies conducted in Kentucky indicate that there is no evidence that commercial preparations of P. popilliae increase the incidence of milky disease significantly in field populations.

Bacillus thuringiensis (Bt)

Bacillus thuringiensis (Bt) is a bacterium that causes disease in several kinds of insects. Recently several new strains of Bt have been identified that are quite specific in the kinds of insects they attack. Because they are more specific, they can be used in a turf setting without disrupting some of the natural enemies, such as predator beetles and mites, which are present.

The strain which is effective against caterpillars (particularly cutworms and sod webworms) is known as the “kurstaki” strain, and is available commercially (some products are not registered for turf use, so check the label carefully before purchasing or using any formulations). The material is formulated to be applied to the turf with a hydraulic sprayer. As with several of the biological control agents currently available, Bt kurstaki is most effective when directed against small stages of caterpillars (less than 0.25” long). Bt kurstaki will not kill caterpillars immediately after exposure, but it will paralyze the gut and digestive system very quickly after the insect ingests the material, so feeding activity stops soon after exposure. Currently, there are no published field trials available reporting the effectiveness of Bt kurstaki for caterpillar control in turf.

A newer strain of Bt (japanensis, or ‘buibui’) appears very promising based on field trials targeting Oriental beetle and Japanese beetle grubs, but looks less effective against European chafers and Asiatic garden beetles. It must be applied when grubs are just beginning to hatch (usually early to mid-August). This strain is not currently under commercial development.

Beauveria bassiana


Beauveria bassiana is a fungus that occurs naturally in New England turfgrass, and is favored by wet conditions in the spring. The fungus is a natural enemy of chinchbugs. B. bassiana is sometimes available commercially, but no field trials have been conducted (and published) to determine whether commercial applications will reduce pest populations.

Entomopathogenic nematodes

Entomopathogenic nematodes are small, microscopic worms which attack insects. Certain species of these nematodes have proven to be quite effective against some of the surface feeding insects (cutworms, sod webworms). The preparations, consisting of live organisms, must be handled more carefully than traditional insecticides. For example, they are temperature-sensitive, so the containers must be stored in areas that will not get too hot (above 80° to 85°) and will not freeze.

Entomopathogenic nematodes can be applied through standard hydraulic sprayers (although very fine filters should be removed). However, these nematodes are very sensitive to desiccation. To maximize their effectiveness, nematodes should be applied in early morning or late afternoon (not between 10 a.m. and 2 p.m.) and should be watered in immediately. In addition, nematodes are most effective when the target insects are small.

There are several species of nematodes which are commercially available in the United States at this time. Steinernema carpocapsae is available as several trade names. Studies throughout the Northeast indicate that this species of nematode is not particularly effective against white grubs, but it can be effective against caterpillars in turf (sod webworms and cutworms). Many turf managers do not rely on nematodes for caterpillar control, however, because they tend to be inconsistent. If nematodes are used to target caterpillars, they must be applied when caterpillars are small (less than 0.25 inch long), and must be watered in with at least 0.25 inch water.

Several field studies have been conducted over the past five years, testing the effectiveness of entomopathogenic nematodes against annual bluegrass weevils (ABW). The results have been inconclusive — occasionally there appears to be a reduction in larval populations in the first generation, but the “window of opportunity” (when larvae are susceptible) appears to be about five to seven days each spring. The nematode showing the most promise is Steinernema carpocapsae, but more field testing must be completed before golf course superintendents can be confident using nematodes for ABW control.

Heterorhabditis bacteriophora (HB) is also available under many different trade names, and field trials in Massachusetts indicate that it can reduce white grub populations when used properly. However, studies conducted at Rutgers University indicate that HB is much more effective against Japanese beetle grubs than other species of grubs. When targeting grubs, nematodes must be applied when grubs are present but still small (often mid August to early September) and must be watered in with at least 0.25 inch of water.

Neem

Azadirachtin is a derivative of the neem tree, which grows in India and other tropical settings. The compound repels several kinds of insects and often causes them to stop feeding. Neem also acts as an insect growth regulator on some insects, preventing them from molting normally from one stage to the next. There is an indication that the neem compounds may sometimes reduce populations of the surface feeders (cutworms and sod webworms). Some commercial applicators report that applications of azadirachtin have been inconsistent. Also, some azadirachtin formulations are very viscous and difficult to handle in a sprayer.

Spinosad

Spinosad (Conserve) is a synthetic preparation of a soil actinomycete which is often categorized as a biological control material, even though it is synthetic and produced in large facilities. It has provided significant control of larval populations of annual bluegrass weevil in the first generation in several field trials. It must be applied as the larvae are emerging from the stems, before they reach their largest (fourth and fifth) instars. Spinosad is also very effective against caterpillars in both turf and landscape setings.

Beneficial insects in the turf setting

If a turf manager were to take a cup cutter sample from a turf area which had not been treated with insecticides for a year or two, and extracted all of the insects and other arthropods from that sample, that manager would be amazed at the diversity of arthropod life in the sample. There are countless beneficial insects and close cousins active in the thatch and upper root zone, decomposing organic matter or preying on other insects. Some of the predatory arthropods include ground beetles (most of which feed on insect eggs and small soft bodied insects, such as small caterpillars or aphids), predatory mites, spiders, and ants. Some of the decomposers include springtails (often in huge numbers), saprophytic nematodes, symphylans, and some mites. While we do not yet fully understand the role of each of these creatures, we do know that their role is crucial to thatch management, and to the overall health of the turf.

Unfortunately, some of the insecticides that are currently used in turf settings are “broad spectrum” materials, which means they kill a wide range of insects and other arthropods, including many beneficial insects. If these broad spectrum insecticides are used regularly, the balance of beneficial insects may change over time, so that fewer of the predators or decomposers remain active. As a result, pest populations can build up more rapidly following an application, because some of the natural enemies have been destroyed and are no longer available to provide a natural level of control of the target pest.

It is critical, therefore, that insecticide applications should be made only when sampling has demonstrated that a pest population has reached the threshold level, and only to areas for which infestation has been confirmed through careful monitoring.

Big-eyed bugs are naturally occurring predators that can be found in many New England turf settings. They are very effective predators of chinchbugs, but unfortunately they look very similar to chinchbugs (except for big bulging eyes), so sometimes insecticide applications are made that reduce big-eyed bug populations. These predators are not available commercially.

Endophytic turfgrass cultivars

Endophytic cultivars have within the seed and plant itself a beneficial fungus called an endophyte. The fungus is unable to live outside of the seed or plant, and so it depends upon the plant for its survival. The plant also benefits from its association with the fungus. Endophyte-enhanced cultivars tend to be vigorous even under conditions of stress such as minimal fertilization and irrigation, and exhibit a level of resistance to foliar feeding insects such as sod webworms, and in particular chinchbugs and billbugs. Endophytes impart no tolerance or resistance to root feeding insects such as white grubs. Furthermore, cultivars which contain endophytes may vary in their susceptibility or in their resistance to disease. This factor is not as clear cut as insect resistance, and will vary by cultivar as well as by disease.

At present there are some endophytic cultivars of perennial ryegrass, tall fescue, creeping red fescue, hard fescue and chewings fescue available. When repairing lawns and other areas that have been damaged by surface feeding insects such as chinchbugs, billbugs, or sod webworms, select endophytic cultivars of appropriate turfgrasses whenever possible. Keep in mind that the percent of endophyte infection in a cultivar may differ by seed lot. Also, the level of endophyte in old or improperly stored seed will decrease as the fungus loses viability. Check with a reputable seed supplier for cultivars with endophyte, as well as for level of endophyte in a particular seed lot.

Endophytic turfgrass cultivars should not be used where animals may graze. The alkaloids produced by the endophyte and the interaction of the endophyte with the plant not only impart some degree of insect resistance to the plant, but also have the potential to sicken animals that may feed upon the plant.