EVOLUTION IN ACTION

American farmers count the corn rootworm amongst the most destructive of their insect enemies. These 2 species of beetle have achieved 2 extraordinary feats of evolution in their battle against agricultural initiatives. The larvae, or “worms” live in the soil, and eat their way into the root systems of corn - on such a scale that crops can be devastated. Yet they will not accept any roots other than maize and related grasses; the larvae starve rather than eat other crops.

In the distant past, the grasslands of the midwest and Mexico were mainly a tall grass known as teosinte, and centuries of cultivation by native Americans led to the evolution of a strain that was to become maize. The 2 species now known as the northern and the western corn rootworms fed only on the roots of perennial wild grasses. Although maize is indisputably an annual, the farmers’ practice of planting corn in the same field year after year removed this obstacle, as overwintered eggs hatched amongst the roots of the new season’s crops. The switch to maize was accomplished; round 1 to the bugs.

Entomologists studying the problem appreciated that the harm to crops could be prevented by a simple crop rotation, and first made this suggestion in 1883. In the nineteenth and early twentieth centuries, corn was grown in rotation with hay, clover, alfalfa, or small grains. The hatching larvae found no food, and the first year corn grew in fields that were free of rootworm eggs; round 2 to the farmers.

But, beginning in the 1920s, soyabean replaced these other crops, and the rotation became more and more an alternation between maize and soyabean, so that by 1990, 67% of Illinois’ corn crop was grown in rotation with soyabean. The northern corn rootworm was the first to find a way to circumvent crop rotation. Instead of terminating diapause after the first winter and hatching when only soyabean is growing, many of the eggs now remain in diapause another season and hatch in time for the next corn. Northern corn rootworms with an extended diapause are spreading and will eventually be prevalent throughout the corn belt. Small proportions wait 3 or 4 years before hatching - an evolutionary tactic that will defeat even irregular crop rotation; round 3 to the northern corn rootworm.

Meanwhile, western root cornworms were finding a different solution. The adult beetles started to lay their eggs in the soil around the soyabean crop. This ploy sounds suicidal, but given the prevailing corn-soyabean rotation, the larvae hatch nicely in time for the fresh corn crop. Round 4 to the western corn rootworm!

This tale of [un]natural selection is but one of a series of fascinating accounts of how bugs find strength in numbers - the subtitle of Gilbert Waldbauer’s wonderful book “Millions of Monarchs, Bunches of Beetles”. Copies are held in Suffolk public libraries.

Waldbauer G. (2000) Millions of Monarchs, Bunches of Beetles. How bugs find strength in numbers. Harvard University Press (p104-109)

Rob Parker