Some microscopic nematodes jump for their supper.
Story by Adam Summers - Illustrations by Sally J. Bensusan
Many creatures-snakes, most fishes, some lizards, and all worms-have mastered the neat trick of getting around without benefit of appendages. Not even the aerial realm is free of the limbless: there are jumping vipers in South America, flying (or, more accurately, gliding) snakes in Southeast Asia, and even worms that fling themselves into the air on occasion. Especially intriguing to Jim Campbell, of the USDA's Agricultural Research Service, and Harry Kaya, of the University of California, Davis, are microscopic (ten laid end to end would just about fit across the top of a pencil eraser) parasitic nematodes. These worms can propel themselves through the air to infect their hosts-sometimes leaping nine times their own body length in the process.
Most nematodes, also called roundworms, are small, simple worms lacking the segmentation of the more familiar earthworms. Many are parasitic, living in the gut, the muscles, and even the circulatory and respiratory systems of vertebrates and invertebrates. Lawn doctors are particularly fond of a couple of species in the genus Steinernema that infect adult mole crickets (bullet-headed cousins of the house cricket). Mole crickets can wreak havoc on lawns and golf courses: the adults dig burrows as big across as your thumb, and the larvae eat grass roots. The native species of mole crickets are relatively benign, but the introduced species (most of which entered North America from South America, Europe, and Southeast Asia in the late 1700s and the 1800s) are real pests, right up there with leaf spot and necrotic ring spot on the list of things you don't want on your lawn.
When a free-living juvenile nematode lands on a cricket, it makes its way inside through the digestive or respiratory system and penetrates the body cavity, where it matures into an adult. Bacteria inside the roundworm go along for the ride. These microbes produce a toxin that is beneficial to the worm but poisonous to the cricket. Several generations of nematodes can live and reproduce happily inside the cricket, but eventually the bacterial toxins overwhelm the cricket, and it dies. With the death of their host, the current cohort of juvenile nematodes (together with their bacterial companions) burrow out of the corpse and lie in wait for new victims. When a cricket happens by, any juvenile worms it touches will try to latch onto it; roundworms that are too far away to climb on board but are within striking distance will hurl themselves into the air and, with any luck, land on the hapless arthropod.
To take to the air, a nematode must leave the peculiar watery realm it inhabits in its free-living stage. Nearly every surface in our environment is coated with a thin film of water. Even seemingly dry surfaces may be covered with a water layer several molecules thick. Jumping nematodes are so tiny that they can live within this filmy world. They pass their youth squirming through it, flopping and writhing across soil particles and blades of grass to get from one place to another. When ready to hitch a ride with an insect, however, a nematode rises up on the tip of its tail with its mouth in the air. "Standing up" decreases the adhesive force of the water surrounding the worm's body, making the worm easier for a passing cricket to pick up. It also, as Campbell and Kaya found, puts the tiny creature in position to execute its aerial acrobatics.
Hit by a puff of cricket-scented air, a nematode rapidly bends over, touching its head to its body about where its knees would be-if it had any. The cohesiveness of the thin film of water covering the worm holds it in this bent posture. Imagine taking a long, thin balloon (the worm) and bending it into an O with a little tail left over. Now, move the balloon so that the point of contact slides "back" toward the bend and away from the tail. This will tighten and close the loop, stretching the surface along its outer edge and causing pressure inside the balloon to rise. As a result, the forces working to straighten the balloon become stronger and stronger. So it is with a nematode that bends and then slides its head back along its body. Eventually the straightening force exceeds the cohesive force of the water holding the worm in its contorted pose, and it suddenly flings itself off into space. In this impressive maneuver, the nematodes jump primarily in the direction of the triggering odor, though they often fall short or miss the target.
Current pest-control practices on golf courses include spraying bucketfuls of live parasitic nematodes onto areas of cricket infestation in the evening, when the grass and soil are moister. Unfortunately, grass suffers from species of nematodes that are not parasitic and that eat plants. The treatment for these undesirable roundworms also kills the useful ones, so some golf-course superintendents are faced with a tough choice: protect their precious turf from the ravages of burrowing crickets or lose some of the course to plant-eating nematodes.