Insects understand silk

Predators not only kill and eat their prey, they scare it away. Often mice leave the house where the cat has settled, and the presence of spiders affects the number of herbivorous insects.

The presence of a spider can be judged by the presence of a web. All spiders produce spider silk; Even those species that do not weave trapping nets leave silk traces on their territory. Ann Rypstra, a professor at the University of Miami, USA, and Christopher Buddle from McGill University, Canada, proved that spider silk actually alerts insects to the presence of a predator and influences their behavior. Frightened pests eat less.

Experiments were conducted in Miami on leaves and whole bean plants ( Phaseolus vulgaris ). To rule out the influence of long co-evolution of predator and prey on the behavior of animals, the researchers chose as herbivorous insects the Japanese bean beetle ( Popillia japonica ) and the Mexican bean ladybug ( Epilachna varivestis ), beetles that have lived in eastern North America for only about 50 years. Adults of both species eat bean leaves and occasionally become prey for spiders. But they did not encounter the Tetragnatha elongate spider, because this species is common in floodplain forests and is not found in fields sown with agricultural crops.

The researchers conducted experiments in the laboratory, on plants and on soil. The bean leaf was evenly wrapped with five turns of spider thread drawn from T. elongate , or silk from the cocoon of the silkworm Bombyx mori . The petiole of the leaf was wrapped in damp cotton wool, the leaf was placed in a Petri dish with a diameter of 20 cm and one beetle was planted there. The control leaves were not wrapped in silk, only touched with hands. A day later, scientists determined how many square millimeters of leaf the pest had eaten.

Under field conditions, the leaves were wrapped in cobwebs or silk directly on the plant and, together with the beetles, were packed into gauze bags measuring 20 by 20 cm. After 24 hours, the leaves were collected and the area eaten was determined. In some cases, the treated leaves were simply left without any bags at the mercy of local herbivorous insects. In this case, the silk treatment was repeated after 3 days and the leaves were collected after 6 days. Finally, the researchers worked with whole plants. All leaves on one plant were processed or simply touched with hands. Two beetles of each species were planted per plant and covered with a mesh cylinder buried several centimeters into the soil. Previously, six leaves were randomly marked on the beans and after three days the areas of damage on them were determined. Some treated plants were left open for anyone to eat. After 3 days they were processed a second time, and on the 6th day the results were analyzed.

And the results in all cases were similar. Both in cups and in the field, the web reduces the area of ​​damage on leaves by 40-50% compared to the control. Silkworm silk also protects leaves from being eaten, but to a lesser extent than spider webs, and in some cases its effect is uncertain. Beetles of both species responded to spider webs and silk in the same way.

The scientists conducted additional laboratory studies with a wider variety of fibers. In addition to fresh cobwebs and silk, they wrapped the leaves with cobwebs pretreated with 70% alcohol, blond hair, and 10-micrometer-thick aramid fibers. Aramid fibers and hair did not affect the preservation of leaves. Spider silk, both fresh and alcoholized, protects them best from being eaten. silk B. mori is less effective.

Consequently, not every fiber on the leaves, namely cobwebs, frightens herbivorous insects. Perhaps they would run away somewhere, but the experimental conditions do not allow this, and frightened pests simply eat less. This effect is not associated either with the behavior of the spider (it was absent) or with the co-evolution of predator and prey.

As always in such cases, scientists are planning further studies with a more extensive list of species. But now they note that spider silk itself has the ability to suppress the activity of potential spider victims. Spider webs have many beneficial properties, and now one more has been added to them - controlling plant pests. Therefore, researchers advise to protect spiders, create favorable conditions for them, populate fields with them, and let them hang their webs everywhere.

Natalia Reznik

Rypstra AL, Buddle CM. 2012 Spider
silk reduces insect herbivory. Biol Lett
9: 20120948.