What do you get when you mix certain pesticides and insect-parasitic nematodes? The perfect killer.
According to Parwinder S. Grewal, an entomologist with the Ohio Agricultural Research and Development Center (OARDC), some insecticides can increase the effectiveness of entomopathogenic nematodes, which are used to control pests such as white grubs and fungus gnats.
"While some pesticides can be toxic for the nematodes, others enhance their parasitic effect," Grewal explained. "The pesticide and the nematodes act in a synergistic way."
Which means the pesticide does not turn nematodes into mutant killers, but rather works as a loyal sidekick while the nematodes combat harmful insects.
Found abundantly in the soil, nematodes are microscopic worms that kill many insect and mollusk agricultural pests. Several species have been successfully used for pest control in citrus, cranberries, mint, strawberries, mushrooms, nurseries, greenhouses and turfgrass in North America, Europe, Japan and Australia. Grapes are expected to join the list next year.
"We've been studying the compatibility of nematodes with other pest control sources over the last few years," Grewal said. "This is a very nice discovery because the lack of compatibility information has been a major impediment to further expansion of their [nematodes'] use."
Together with colleagues from Rutgers University, New Jersey, and the University of California at Davis, Grewal discovered that the insecticide imidacloprid and the nematodes Heterorhabditis bacteriophora and Steinernema glaseri interact synergistically against turfgrass's biggest nemesis: white grubs.
White grubs are the root-feeding larvae of scarab beetles (such as masked chafers, the Japanese beetle, and the oriental beetle), which cause significant damage to many agricultural and horticultural plants.
One of the most widely used agents to destroy white grubs is imidacloprid -an insecticide found in products such as Admire, Condifor, Gaucho, Premier, Premise, Provado, and Marathon. However, the efficacy of this insecticide declines as the white grub develops, so it has to be applied preventatively to large turf areas that may or may not be infected.
Entomopathogenic nematodes, on the other hand, are effective against the later stages of the white grub. After entering the body cavity of insects through a variety of openings, nematodes release bacteria that multiply and kill the host within three to four days. The nematodes then feed on the dead insect, reproduce and migrate in search of new hosts.
Nonetheless, white grubs have developed a series of behavioral, morphological and physiological barriers to infection during their co-evolution with nematodes, which makes it harder for the worms to penetrate their hosts. That's where imidacloprid comes into play.
"Imidacloprid disrupts white grubs' normal nerve function, breaking down the defenses that they display in response to nematode attack," Grewal explained. "This helps the nematodes get inside the grubs and start the parasitic cycle."
Since the later white grub stages are easier to detect, Grewal suggests applying an imidacloprid-nematode mix to infected turf. This combination reduces costs and provides a safer and more environmentally friendly alternative to more hazardous agrochemicals used for turfgrass pest control.
Grewal has also studied the resistance of the nematode Steinernema feltiae to fungicides such as Nimbecidine, cynnamaldehyde (Cinnamate), ZeroTol and azoxystrobin (Abound). Steinernma feltiae is used for the control of fungus gnats.
"It is usually more economical to tank-mix nematodes with one or more inputs for application." Grewal said. "Nematodes are tolerant to short exposures of most fungicides, but in some cases these chemicals can reduce nematode viability and virulence. That's why we needed to know for sure which fungicides were safe."
Grewal concluded that Nimbecidine, Cinnamate and ZeroTol were toxic to nematodes. Meanwhile, Abound and neem (the active ingredient in Nimbecidine, used in this case as pure oil without any additives) caused no harmful effect on the nematodes.
"The more we learn about the interaction of nematodes with agrochemicals, the better we are equipped to offer safer and more cost-effective insect management methods," Grewal remarked. "With this knowledge, nematodes are more likely to be accepted by growers as an effective way to fight pests."
Entomopathogenic nematodes have been mass-produced since the mid '80s and their use is becoming more popular. Florida orange growers, for instance, apply nematodes to 50,000 acres each year to help control the citrus root weevil. Unlike pesticides, nematodes continue working for quite some time after the first application -a single host can produce 300,000 to 400,000 new nematodes, which then seek additional victims.
Grewal's research on the compatibility of nematodes with pesticides was funded by an OARDC grant. OARDC is the research arm of Ohio State University's College of Food, Agricultural, and Environmental Sciences.