by Tamara Scully

Surveys suggest that an increasing percentage of greenhouse growers are utilizing biocontrols to combat pest issues. An informal survey of New York state growers found that 40% were using insecticides alone to control fungus gnat. Sixty percent, however, were using some form of biocontrol, including 42% who were using nematodes. For thrips, another problematic greenhouse pest, 53% of the growers were using biocontrols, with 23% of those including nematodes in their arsenal.

John Sanderson, professor of entomology and greenhouse pest management at Cornell University, shared this insight in a webinar, “Using Nematodes in the Greenhouse.” Benefits of using biocontrols such as nematodes include almost no risk of resistance, no restrictive re-entry periods and no post-harvest interval, he said.

Using Nematodes for Insect Control

Nematodes are a broad class of microscopic worms, also known as roundworms, which live in soil. They are believed to be the most abundant creatures on Earth. Some nematodes are plant or animal pathogens, causing disease in susceptible hosts. Others are entomopathogenic (parasitic to a variety of insect). Some are saphrophytic, recycling nutrients in the soil.

It’s the entomopathogenic nematodes which can target and kill some insect pests that are important to growers. These nematodes have at least one lifecycle stage that is reliant upon susceptible insect hosts for survival. Nematodes belonging to the Steinernema feltiae, Steinernema carpocapsae and Heterorhabditis bacteriophage species are parasitic and use plant pathogenic insects as hosts. The Steinernema species are used primarily in greenhouse settings, while the Heterorhabditis can be used but are more commonly found in outdoor growing applications, Sanderson said.

It is the infective nematode juveniles which enter the host through any opening and cause sepsis, leading to its demise. All other lifecycle stages are completed within the host cadaver, until the next infective juvenile stage, at which time the nematode returns to the soil to await a new host.

Nematodes are now sold as biocontrol agents and are commonly packaged in trays by the millions, or in sponges. The sponges have no carrier, so there are no concerns about their use in certified organic operations. The gel in the trays may or may not be certified organic. Nematodes are also available in vermiculite or other substrates. Manufacturers have their own brand names for products containing nematodes.

“If you open the package, you really need to use all of the nematodes right then,” Sanderson said, as they won’t survive long outside of a host.

Products are applied via full volume sprayers, or through boom irrigation. Because living nematodes are needed, they must be kept refrigerated and warmed up slowly prior to use. It’s important to keep the psi of the sprayer to below 300, to use water that is not too hot and to agitate the sprayer tank during application, preventing settling of the nematodes and ensuring consistent application rates. Sprayers must have filters and screens removed to prevent clogging.

In order to properly apply nematodes, soils must be wet – they require a moisture film. The water they are mixed in must be less than 100º F, and the mix must be agitated regularly throughout application. Tanks which previously contained pesticides should be avoided, or the nematodes may be killed.

Two commercial application methods include a five-gallon aeration bucket, complete with a pump which keeps the nematodes suspended in solution and provides them oxygen, or a 50-gallon Ferticart, which also provides agitation. “Most sprayers are useable” for nematode applications, Sanderson advised.

Targeting Greenhouse Pests

Nematodes are used as a preventative measure and should be employed prior to an issue occurring. Some common insect pests in the greenhouse setting can be controlled with nematodes, while others are best controlled with different biocontrol agents, although nematodes often can work with other bugs to enhance protection. Fungus gnats, Western flower thrips and shore flies are the three greenhouse pests primarily controlled by nematodes.

Fungus gnats don’t harm a plant as adults, but the larvae feed on the roots. The Western flower thrip’s pupal stage lives in the soil. Nematodes, however, don’t work on thrips which don’t pupate in the soil. It is not clear if onion thrips are a target insect or not, however.

The adult shore fly vectors root rot diseases on its body, causing problems for the greenhouse grower. Controlling algae – a food source for the flies – will control the shore flies. But nematodes can help keep things under control.

In order to effectively use nematodes to target these pests, greenhouse growers need to pay attention to environmental parameters. Mediums must be moist. Ultraviolet light is harmful to the nematodes, so application should occur in the early or late parts of the day. Nematodes are highly susceptible to warm temperatures.

The temperature, irrigation method and substrate all impact the success rate of nematodes, and will also affect the needed application frequency. Nematodes do best in peat-based substrates. Others, such as rockwool, make it much easier for the nematodes to be washed away during irrigation.

“The irrigation method can really make a difference with how often you have to apply nematodes,” Sanderson said. Drip or overhead irrigation can wash nematodes right out of growing media.

Tests on fungus gnat populations show that in general greater than 80% mortality for up to two weeks is achieved with nematodes. After two weeks, the nematodes are not as effective. Sanderson recommends applications of nematodes at less than two week intervals to maintain control.

Different nematode species will have differing heat tolerances. Graduate student Anna Giesmann conducted studies on fungus gnat populations when nematodes were used under varying temperatures, either 75 or 55 degrees. Two nematode drenches, two weeks apart, and an untreated control were examined.

Plants in the control were found to be highly infected with fungus gnats, while those treated with nematodes in the warmer temperature showed a 40% decline in the fungus gnat population. Those treated with nematodes in the cooler environment had a 60% decrease in fungus gnat population compared to the control. Fungus gnats themselves don’t like cooler temperatures – but the S. feltiae nematodes do.

In another trial, hotter temperatures – 86, 95, 104 and 113 degrees – were used. Three strains of each of the three greenhouse entopathogenic nematodes were used, and were examined after one, two, four, six, eight and 10 hours. H. bacteriophage showed 100% mortality at 104º after four hours. S. carpocapsae survived at temperatures of 113º, while S. feltiae was not able to tolerate warm temperatures at all, and died at 95º.

“High temperatures can make a big difference” in selection of nematodes, Sanderson said.

Nematodes are also able to work with some other arthropods for more complete control of insects. They can work with predatory bugs and fungi, and can work alongside the predatory mites which attack thrips on foliage. It is also possible to grow your own nematodes, rather than rely on commercial preparations.