Nematode worms are tinny but they can infect and kill insect pests using a toxin synthesized by a bacteria from their guts on the dead body of the insect.

After that, the worms feed on the dead insects.

A team at the University of Warwick has found how the toxic protein behaves like a cunningly designed poison pill box that could now be employed as a basis for a totally new range of natural insecticides.

The bacterium's toxic protein (named XptA1) not only kills the target insect but impedes other predators from eating the body, allowing the nematode worms to consume it, multiply and move on, but scientists did not know how it did that.

The research team could reveal the shape of the protein XptA1 and found many traits that make it an especially efficient natural insecticide and possible alternative to some commercial insecticides that are facing higher resistance in the insect populations they target.

The protein was found to be formed from four subunits in the shape of a hollow box which is configured to bind well to part of a caterpillar's gut called "Brush Border Membrane Vesicles" (BBMV).

The XptA1 protein seemed to specifically attack the BBMV of caterpillars Pieris brassicae (cabbage white butterfly, one of the most common species).

The hollow shape permits the molecule to behave as a receptacle for two other proteins (in this case XptB1 and XptC1), forming a poison "complex" which increases the toxicity of XptA1 300 times more to the caterpillars than it would be by itself.

The box shape of the XptA1 protein possibly also helps defend the protein complex from the attack of the acids in the insect gut.

While XptA1 was highly specific to the cabbage white butterfly caterpillar, other variants of these toxic proteins (such as XptA2) targeted other species.

"This research gives us crucial new insights into a family of naturally occurring proteins that are toxic to a number of insect pests. They offer an alternative to current commercial protein based insect toxins have been in use for 40 years and are now starting to meet some resistance. This potential new family of protein based insecticides would overcome such resistance as they operate in an entirely different way." said Dr Sarah Lee from the University of Warwick.