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Steinernema scapterisci nematodes emerging from a mole cricket they have killed.
Steinernema scapterisci Nguyen & Smart (STEIN-er-NEM-a scap-te-RISK-ee) is a species of steinernematid nematode native to South America. Living specimens were obtained in Uruguay in 1985 and brought to Gainesville for testing by members of the UF/IFAS mole cricket program. This nematode was an undescribed species until 1990, when a species name and description for it were published. Preliminary information about it was published before 1990 under the name “a Uruguayan strain of Neoaplectana carpocapsae” because, in fact, it was at first identified incorrectly as belonging to the widespread species Steinernema carpocapsae, then known as Neoaplectana carpocapsae.
There are three major forms in the life cycle of Steinernema scapterisci: egg, juvenile (four stages), and adult (male and female). The third-stage juvenile is normally the only form inhabiting soil, because the other stages occur inside a host. If third-stage juveniles succeed in finding and getting inside a host, they develop to become adults, the adults mate, the females lay eggs, and juveniles develop to the third-stage. At this point, one of two things can happen: (1) the host has died and the third-stage juveniles are released into the soil, where they will await another host, or (2) the host may be close to death, and the nematodes undergo another generation inside it, releasing third-stage juveniles into the soil. Thus, there can be a short cycle, in which children of the founding juveniles are released back into the soil in six to seven days, or there can be a long cycle, in which grandchildren of the founding juveniles are released back into the soil in eight to 10 days. The choice between long cycle and short cycle depends upon nutrient supply available to the nematodes. The actual number of days the cycles take is influenced by temperature — longer at lower temperatures, shorter at higher temperatures. The example given here is at a constant temperature of 24°C (75°F).
If third-stage juveniles newly released into the soil do not succeed in finding and getting inside a host, then eventually they will all die. They will not die all at once, but spread out over time. In one experiment in which containers with nematodes were buried in the soil in winter and dug up 10 weeks later, 16 percent of the nematodes were still alive. Survival might have been poorer at summer temperatures. Survival might also have been poorer if predatory mites or insects had had access to the nematodes.
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The nematodes move about very little in soil. They are capable of slight movement toward and away from the soil surface. However, they basically must depend upon a potential host insect moving very close to them in the soil. They can be dispersed to new sites by host insects before those hosts become so sick due to the infection that they cannot move. For example, infected adult tawny and southern mole crickets can fly at least a mile, perhaps more, and when they die they release third-stage juvenile nematodes into the soil.
Third-stage juvenile nematodes get into host insects through the mouth of the insect or through the spiracles. From the digestive system or the respiratory system of the host, they have to break through to the body cavity. Once inside the body cavity they release specialized bacteria from their own digestive systems. These bacteria multiply in the host, killing it. The nematodes then feed on the large numbers of bacteria now within the host.
Initial tests showed that S. scapterisci killed all tawny and southern mole crickets exposed to it, but only about 75 percent of shortwinged mole crickets. The same tests showed that all house crickets, Acheta domesticus, exposed to it were killed, but only a small or very small percentage of various other species of insects. Furthermore, insects that do not burrow through soil will not be exposed to it in nature. Therefore, house crickets in reality are not at risk because they live in and around buildings rather than in the soil of pastures and golf courses. It does seem, from results of initial tests and of later tests, that shortwinged mole crickets are less susceptible than are tawny and southern mole crickets. Further, mole cricket nymphs are less susceptible than are adults, and small nymphs are scarcely susceptible at all. The tests showed that S. scapterisci is quite specific to Neoscapteriscus mole crickets. Furthermore, results of field tests in pastures in Alachua County failed to show any effect of S. scapterisci on populations of northern mole crickets. Note that northern mole crickets have their own, native species of entomopathogenic nematode, Steinernema neocurtillis.
Steinernema scapterisci was brought to Florida and released in the hope that it would reduce Neoscapteriscus mole cricket populations. Whether it would function as a classical biological control agent or as a biopesticide was unknown at the time. It has proven to have some of the attributes of both. First, when released in pastures, it established and maintained populations for at least five years, killing mole crickets week after week for the whole time. Second, when released in larger numbers in larger plots, it achieved the greatest effect on mole crickets within the first seven to 10 days, with much reduced effect thereafter.
1989-1990 releases in 8 counties
1990-1992 releases at 28 golf courses in 16 counties
1985-2001 total non-commercial releases in 28 counties
Nematode applications in pastures in 2010.
Therefore, Steinernema scapterisci has a future as a classical biological control agent. Populations of it are now established in small areas of many counties in Florida. In time, they will spread. Although the median measured rate of spread is just a few yards a month, in a long time this nematode will populate most or all of Florida, with an enormously widespread effect on mole cricket populations (and some nematodes, carried by infected mole crickets, will enable populations to spread faster). It will have its greatest effect where mole cricket populations are highest, because there it will be able to reproduce best. This means that its effect in pastures, where mole cricket populations tend to be high, is likely to be greater than its effect in turf, where populations tend to be lower.
Steinernema scapterisci also has a future as a biopesticide. It kills mole crickets within seven to 10 days as contrasted with a few hours in many chemical pesticides, but death of infected mole crickets is certain. Although the proportion of mole crickets killed by direct application (in water, at 800 million nematodes per acre) may range up to about 60 percent, and this is not as high as are killed by the most effective current chemical pesticides, the nematode has a special attribute: it has a residual effect. This residual effect has two causes: (1) nematodes can persist in the soil for many weeks, and (2) nematodes that succeed in finding host mole crickets will reproduce, releasing more third-stage juveniles into the soil to infect and kill more mole crickets. This residual effect can continue indefinitely, though kill of mole crickets after the first seven to 10 days will be at much lower levels than it was initially.
Steinernema scapterisci is now being sold commercially in Florida as a biopesticide under the trade name NematacTM S. When applied, the nematode must be irrigated into the ground because it will die if left on the surface: it is susceptible to desiccation and to ultraviolet light. Application in the evening, when light intensity is lower, helps to reduce exposure to ultraviolet light, as does application on cloudy days. Application during rain helps to avoid the need for irrigation. Farmers who want to apply the nematode to pastures where irrigation is not possible should use subsurface application. Note that as of the end of 2010 two such application machines are available to Florida ranchers (one based at Hastings, the other at Ona). To reduce costs, farmers may also apply the nematode in separated swaths, expecting populations of the nematode, in time, to fill in the untreated areas; treat one, skip seven (one treated swath, seven untreated swaths, one treated swath, etc.) has been tested experimentally on a ranch in Polk County and works well due to the rate of spread of the nematodes. This reduces cost of materials to one-eighth of what it otherwise would be. The concept is explained in the UF/IFAS publication “Timing the application of beneficial nematodes to mole cricket activity on pasture to optimize control,” which is based on research described in an article of the Florida Entomologist, “Control of pest mole crickets (Orthoptera: Gryllotalpidae) in bahiagrass pastures with the nematode Steinernema scapterisci (Rhabditida: Steinernematidae).”
Quality of the nematodes to be applied is of great importance. They must be healthy, and they must contain the necessary bacteria in their guts. Health of the nematodes may most readily be determined if they are seen to move, but a microscope is necessary to see them. Their bacterial complement is harder to evaluate, and this is best done by exposing mole crickets to a sample of them in a bucket of soil. If the mole crickets die, then the nematodes can be presumed to have enough of the necessary bacteria. The check for the test of bacteria is another bucket of soil and mole crickets, to which no nematodes are added. These mole crickets, of course, should live, in contrast to those exposed to the nematodes.
Some commercial companies may sell Steinernema nematodes other than S. scapterisci as biopesticides for use against mole crickets. If the label does not tell you the species of nematode, then ask the supplier to give the name in writing. It is true than other Steinernema species will kill mole crickets, at least to some extent, but do not expect them to give any residual activity because they are not known to reproduce in mole crickets.
Nematicides were used on golf courses against plant-parasitic nematodes and doubtless would kill Steinernema scapterisci. However, such chemicals no longer are labeled for that purpose by U.S. Environmental Protection Agency. Studies in the early 2000s showed that Steinernema scapterisci is remarkably resistant to chemical insecticides. The effect of chemical herbicides on this nematode is unknown.
Although the initial cost of applying nematodes to turfgrass areas is high, and sometimes even higher than the cost of chemical nematicides, the eventual cost is lower. As mentioned, the nematode has a residual effect and will kill mole crickets long after chemical insecticides have ceased to do so. The immediate benefit to the turf manager will be lower chemical costs. A second benefit is that the nematode does not pollute the environment with chemical runoff or fumes. There is no cleanup required by local sewage systems and no accidental kill of nontarget organisms. The use of biological control agents instead of chemicals not only saves energy costs, but enhances the reputation of the turf manager in the community.
2001 releases in 20 counties