Know your nematodes

Once you understand the species and their application methods,
nematodes can be used to control turfgrass insect pests.

Randy Gaugler, Ph.D.

With its skin removed, this white grub carcass reveals a tangle of Heterohabditis bacteriophora nematodes feeding inside. Note that the infection has changed the grub from white to red.

Most golf course superintendents know nematodes as troublesome turf pests, but some species of this little parasite can actually benefit turfgrass. Insecticidal nematodes are lethal to white grubs, chinch bugs, web worms, cutworms, bill bugs and mole crickets (5,6,7,9).

Yet conventional wisdom among golf course superintendents holds that "nematodes don't work." Nematodes don’t behave like chemicals, but they are often judged against chemicals on the golf course.

To achieve success with nematodes, superintendents must be able to obtain the right type of nematodes, care for them properly, apply them correctly and then wait for results.

What is a nematode?
Nematodes are simply round worms. Tiny, colorless, unsegmented and without appendages, nematodes may be free-living or parasitic. The parasitic species cause many important diseases of plants, animals and humans.

Yet as insecticides, nematodes are so safe that they are exempt from government registration and residue-testing requirements. No protective clothing is required during applications. Microbial degradation, organic matter and soil pH do not affect nematodes, and they are compatible with most fertilizers and chemicals.

Nematodes are sold for control of soil insects in Europe, China, Japan, Canada and the United States. They are mass produced, with some facilities featuring 20,000-gallon stainless-steel fermenters and centrifuges costing millions of dollars.

The right nematode
Unfortunately, the most readily available nematodes aren't necessarily the most effective against every turf pest. As a result, many superintendents have become skeptical about the effectiveness of nematodes.

The nematode Steinernema carpocapsae is widely available, has a relatively long shelf life (for a nematode) and is effective against many caterpillars infesting turf. But it is ineffective against white grubs because S. carpocapsae is an "ambusher" that simply sits and waits for highly mobile surface insects (such as cutworms and webworms) to pass by it (6). This strategy doesn't work with white grubs because they're relatively sedentary soil-inhabiting insects. Even so, some companies have aggressively marketed S. carpocapsae for grub control, even placing white grub photos on their packaging.

By contrast, the nematode species Heterorhabditis bacteriophora is a highly mobile "cruiser" that is well suited for locating sedentary insects within the soil. Unlike S. carpocapsae, this species has been found parasitizing grubs in nature. An analysis of 82 field trials concluded that "H. bacteriophora can provide control comparable with chemical insecticides" when used in the fall with irrigation, whereas S. carpocapsae was ineffective under any range of conditions (5). Subsequent studies have confirmed that "all strains of H. bacteriophora reduce larval populations to a level comparable with that achieved by use of bendiocarb" (9).

Stability
So why isn't H. bacteriophora widely available for white grub control? Because H. bacteriophora is unstable -- it is difficult to keep alive until it can be put to use on golf courses. It requires special handling or it will perform poorly or inconsistently. Thus, the species is poorly suited in the conventional commercial insecticide market, where a room-temperature shelf life of six months is required.

Instability is not necessarily a fatal flaw in a commercial product. The dairy industry has been hugely successful in selling low-stability, high-quality products. Similarly, the new microbrewery industry has demonstrated there is a strong market niche for "fresh" fermented beverages sold without preservatives. The key has been local production and quick turnover.

Most of the expense in producing nematodes involves formulation, storage, transport and waste disposal -- costly steps that could be made less expensive by developing novel, small-scale, disposable fermentation technology that would permit local production. More importantly, this would mean that only "fresh" nematodes would be applied.

Hundreds of thousands of nematodes spill from a dead caterpillar in search of fresh insect hosts.

The Rutgers Center for Turfgrass Science is developing microproduction techniques of nematodes with support from The GCSAA Foundation and the Tri-State Turf Research Foundation.

Nematodes aren't chemicals
Nematode products resemble chemical insecticides in appearance and application methods, but as biologicals they are "living organisms that require specific conditions to be effective" (10). Insecticidal nematodes can't withstand desiccation or ultraviolet light, so they must be watered into the soil after application. Pretreatment irrigation is also recommended if the soil is dry. Chemical insecticides applied against soil insects in turf have the same prerequisites (1), but mostly for different reasons. The negative impact of ignoring irrigation recommendations, however, is more severe for nematodes.

In addition, nematodes are effective within a narrower temperature range than are chemicals, and they are also affected more by suboptimal soil type, thatch depth and irrigation frequency (5).

Inferior storage and handling will inactivate nematodes, whereas chemicals are more forgiving of similar mistreatment. Nematodes do not withstand hot vehicles any better than the family dog, and they don't store well. They're most effective when used within a few days of purchase.

Furthermore, they can't be left in spray tanks for long periods because they can't tolerate settling or insufficient oxygen. Certain species can't be applied with high-pressure application equipment, and unused nematodes can't be applied the following year. Different nematodes may also require different screen sizes. In short, nematodes require a more knowledgeable user to achieve acceptable results.

Education is the key
In addition to the usual environmental and safety issues, chemicals also have performance problems in soil (1). But chemical use is supported by a large information and education base. No comparable database exists for technology transfer of any biological agent. The insecticidal nematode industry lacks the salespeople and consultants that the chemical industry boasts, and researchers are rarely provided financial support to test and gain experience with the emerging new nematode technology.

In fact, many biocontrol workers interested in field efficacy tests with nematodes must buy nematodes from producers -- circumstances that are unthinkable in the chemical industry. Is it any wonder that even educators tend to have limited expertise with nematodes? The problem is aggravated by "limited communication between research and extension" on biologicals (11).

New biological control technologies such as nematodes will continue to be underutilized until more people understand how to use them effectively. To address this issue, Rutgers University is leading a 10-state multimedia effort to develop instructional aids on insecticidal nematodes. Those aids include videos, Web sites, wall posters, fact sheets and a slide set. Turf will be a major focus of this ambitious educational project. The training aids will be used in an intensive workshop intended to create a multistate cadre of instructors who are extension personnel. Participants will transfer their new knowledge to nematode users within their home states through training sessions and seminars.

Contact chemicals have conditioned growers to evaluate control based on quick action, but biological control agents work more slowly. This disadvantage may be offset by the fact that nematodes may be recycled to provide longer-term control.

For example, in an Ohio golf course study that examined H. bacteriophora recycling, an initial white grub reduction of 60 percent had increased to 96 percent after 8 months (7). This deferred value, similar to an investment, requires very different thinking from users accustomed to fast-acting chemical agents. Education will be pivotal to helping users recognize that evaluation, storage and application methods are different for nematodes.

Future technology
Education will improve results, but nematode technology must improve as well. Fortunately, it is. Research in dozens of laboratories around the world is being directed toward nematode production, formulation and application methodology. Although there is room for improvement, significant advances have been made over the last five years to close the gap between nematode- and chemical-based insecticides in price, ease of use and efficacy.

The most powerful tool for endowing biologicals with some of the positive characteristics of chemicals is molecular biology. At Rutgers, we have pioneered genetic engineering of insecticidal nematodes. These nematodes are ideal for genetic manipulations because of their simplicity, transparency, ease of culture, short generation time and small size. We used microscopic glass needles to inject foreign genes into the ovary of H. bacteriophora (3). Subsequently, we developed a novel method using silicone chips covered with large numbers of microscopic pyramid-like projections coated with foreign DNA. To easily identify our engineered nematodes, we injected a gene from jellyfish that makes these nematodes glow with a green fluorescence.

Once armed with reliable molecular techniques for engineering our nematodes, we brought these tools to bear on a key obstacle to nematode use -- instability. Because high temperatures in warehouses or during transport frequently deactivate entomopathogenic nematodes, our initial efforts centered on using heat-shock proteins to improve nematode heat tolerance. Produced when organisms are exposed to high temperatures, these compounds enable cells to repair heat damage. We injected a heat-shock protein gene from a free-living nematode, Caenorhabditis elegans, into H. bacteriophora. Offspring of the injected nematodes took up the foreign DNA, increasing the gene number from one to 10. This "overexpression" of heat-shock protein conferred permanent heat tolerance, resulting in transgenic nematodes that are 18 times better than unmodified nematodes at surviving high-temperature stress. Laboratory risk-assessment studies have revealed no changes in these nematodes other than enhanced heat tolerance.

Regulatory constraints have hindered the development of some genetically engineered organisms. Insecticidal nematodes, however, possess a unique niche in the regulatory environment. In 1996, we readily obtained approval at federal, state and local levels to release our transgenic strain into turfgrass field plots at the Rutgers Upper Deerfield Experiment Station in Salem County, N.J. (4). Results from the study support the regulatory view that the transgenic nematode strain is an unlikely environmental threat.

Genetic engineering is no panacea. Not all nematode limitations are resolvable with a technology fix -- user knowledge concerning biologicals is not amenable to engineering. Nevertheless, genetic engineering clearly shows early promise, and we anticipate that molecular biology will be exploited at an accelerated rate for improving nematodes as biological insecticides in turf.

Twenty years ago, only about a half dozen scientists understood nematodes' potential for controlling insects. Last year, 60,000 acres of citrus were treated for root weevils using nematodes. Someday, similar inroads may be made in the turfgrass industry. This will require further technological advances and recognition by users that they'll have to learn how to use the new technology. We may also have to tolerate less-than-perfect turfgrass as we employ less toxic remedies in the turfgrass environment.

Literature cited
1. Brandenburg, R.L. 1995. Effective management of subsurface turf pests. Golf Course Management 63(12):61-64.
2. Dysart, J. 1995. Disarming directions: Today's golf course management practices have muted some of the industry's loudest critics. Golf Course Management 63(12):25-26.
3. Gaugler, R., and S. Hashmi. 1996. Genetic engineering of an insect parasite. pp. 135-57. In: J. Setlow (ed.), Genetic engineering principles and methods, Vol. 18. Plenum Press, New York.
4. Gaugler, R., M. Wilson and P. Shearer. 1997. Field release and environmental fate of a transgenic entomopathogenic nematode. Biol. Contr. 9:75-80.
5. Georgis, R., and R. Gaugler. 1991. Predictability in biological control using entomopathogenic nematodes. J. of Economic Entomology 84:713-720.
6. Kaya, H., and R. Gaugler. 1993. Entomopath-ogenic nematodes. Annual Review of Entomology 38:181-206.
7. Klein, M.G., and R. Georgis. 1993. Persistence of control of Japanese beetle (Coleoptera: Scarabaeidae) larvae with steinernematid and heterorhabditid nematodes. J. of Economic Entomology 85:727-730.
8. Office of Technology Assessment. 1995. Biologically based technologies for pest control. Rep. No. OTA-ENV-636. U.S. superintendent of documents, Washington, D.C.
9. Selvan, S., R. Gaugler and J. Campbell. 1993. Efficacy of entomopathogenic nematode strains against Japanese beetle, Popillia japonica, larvae. J. Economic Entomology 86:353-359.
10. Shetlar, D. 1992. The biological control of insects. Golf Course Management 60(4):92-98.
11. Waage, J. 1997. Biopesticides at the crossroads: IPM products or chemical clones? pp. 11-19. In: H.F. Evans (ed.), Microbial insecticides: novelty or necessity? British Crop Protection Council Symposium Proc. No. 68.

Randy Gaugler, Ph.D., is a research professor at Rutgers University in New Brunswick, N.J.