A worm's eye view of biocontrol

credit: Patricia Stock

A handful of soil contains thousands of microscopic worm-like organisms known as nematodes. Many of the more than 25,000 species so far classified worldwide - out of an estimated total of half a million to a million - cause significant damage to crops such as tomatoes, potatoes, bananas, rice and spinach. But not all nematodes attack crops. Some attack instead the soil-inhabiting insects that destroy crops, thereby providing non-toxic and non-chemical pest control.

Dr Patricia Stock, an assistant professor in the Plant Pathology and Microbiology Department of the University of Arizona, USA, is working in Costa Rica, Arizona and Jordon to develop the use of local entomopathogenic nematodes (nematodes that parisitise insects, or EPN) as a mechanism for biological control.

EPN are found in many ecosystems, from deserts and grasslands to rainforests. In Jordon and Arizona, they are already used to control weevils in palm heart and fruit trees and, in Colombia, Ecuador and Cuba, to protect coffee plantations. EPN can also kill scarab beetle and such moth larvae as the corn earworm and cutworm, which can cause extensive damage to crops such as coffee, cocoa and maize. "In all these countries," says Stock, "these pests cause major problems," particularly as they can feed on a wide range of plants. Researchers are experimenting with EPN that kill their destructive cousins, the root-knot nematodes that plague horticultural crops.

Dirty work

credit: Patricia Stock

In the soil, the juvenile EPN enters an insect larva through a natural orifice or by penetrating the shell. Once inside, the EPN vomits up E. coli-like bacteria that kill the larva within a day or so. The nematodes are then able to use the corpse for the completion of one or more life cycles. The "outside form" of the EPN, which can withstand dry conditions for long periods if necessary, leaves the corpse by the thousand to begin the process again.

Pest control using nematodes is desirable as it works more quickly than insecticides - and does so without the chemicals' negative effects. "Crops and plantations are surrounded by natural ecosystems that have been loaded over the years with chemical pesticides," says Stock. "There is a great deal of soil degradation." Local nematode preparations not only provide chemical-free control of pests, they are cheaper than either pesticides or the commercial preparations of exotic nematodes that have been around for about 20 years. Stock stresses that custom-made formulations using local nematode species are better than those using exotic species.

"Local EPN may be better adapted than exotic species or strains to local pests and environmental conditions," she explains. "And they do not compromise the local biodiversity of pathogens, parasites and other beneficial insects." Stock is comparing commercial and local preparations with regards to their ability to kill local pests and handle climate conditions.

Natural pathogens

To find which nematodes and their accompanying bacteria could be used for local crop-pest management, Stock and her colleagues are searching for nematodes known to be parasitic to insects and those that are not yet confirmed to be insect pathogens. "We have identified new species all around the world," she says. "Some of the latest are Steinernema scarabaei, a natural pathogen of the Japanese beetle in New Jersey." Several others whose hosts are not yet known include Steinernema hermaphroditum from Indonesia, Heterorhabditis downsi from Ireland and Steinernema anatoliense from Turkey.

Once researchers find and isolate a particular nematode species, they confirm its identity through examination of the body morphology and DNA analysis. Researchers then can multiply large numbers, which are stored in a gelatin suspension or powder. "Farmers are provided with foam pieces containing high concentrations of nematodes," Stock explains. "The farmers soak them in water and then use the water to spray the fields." Farmers use standard spray equipment but need not use the safety equipment required for spraying pesticides.

Countless possibilities

Colombia and Honduras are the lead countries in using locally developed EPN sprays but, as Stock observes, "Many other countries are becoming more receptive to these options." She believes that there will be no problem developing local commercial sprays "if we give the nematodes to the right company. In the US, there are many already involved and very enthusiastic."

Stock believes the study of EPNs offers countless possibilities in terms of understanding the basic science of host-pathogen interactions and the control of crop pests. "There are more than 50 species of entomopathogenic nematodes described so far," she observes. "If it doesn't work with one nematode, you can move on to the next." In addition to crop-pest control, nematodes are currently being examined as a way to control ticks and lice in grassy areas of Israel.

Article by Treena Hein