Undercover agents

What makes nematodes so good at killing crop pests

By Sanjeev Kumar Kanchan
Published: Saturday 15 May 2010

with chemical pesticides accumulating in crops, entering the food chain and creating pests with the next level of immunity, biocontrol is taken a little more seriously. Bacteria, fungi and insect parasitic nematodes assume greater importance in agricultural research.

The Commonwealth Scientific and Industrial Research Organization was the first to use nematodes commercially against black vine weevils and currant borer moths. These slender, 2.5-mm-long roundworms are piquing the interest of biocontrol specialists the world over because their use does not require any safety equipment such as masks. They do not contaminate groundwater, leave behind toxic residues or endanger the lives of pollinators. In Brazil, Venezuela and tropical areas of Australia they have successfully been applied to curb the stem weevil disease in banana trees.

The roundworms enter the insect’s body either with its food or penetrate the skin. These worms sustain symbiotic bacteria inside their intestines. Once inside, they live parasitically off the host’s body and also release the bacteria.

The microbes then proliferate rapidly and kill the insects. Both symbiotic partners feed on the dying insect. This, however, is not a new discovery; only the mechanism of action of the bacterial toxins was elusive, said Klaus Aktories, from the University of Freiburg in Germany and one of the lead scientists of the group that researched on it.

Researchers from the University of Freiburg and the University of Bochum in Germany in collaboration with Dow Agrosciences in usa studied the insecticidal activity of a nematode and its associated bacterium Photorhabdus luminescens that acts on the honeycomb moth (Galleria mellonella).

When this nematode invades the honeycomb moth’s larvae, the bacteria produce two toxic protein complexes. The scientists tested the efficacy of the toxins on the moth’s blood cells and HeLa cells derived from cultures of cervical cancer. The toxins caused the structural proteins of the host cells’ cytoskeleton to roll up into long strands. Since it is the cytoskeleton that holds the cell in place, the cellular infrastructure collapses and its immune system looks on helplessly at the proliferating bacterial invaders. The study was published in Science on February 16.

C Sankaranarayanan, a nematologist with the Sugarcane Breeding Institute at Coimbatore in Tamil Nadu, said: “P luminescens produces many chemicals which possess antifungal, antibacterial and anticancer characteristics. With emerging resistance to Bt crops among insects, it definitely is the next generation microbial pesticide.”

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