Insects and Wildlife

Insects and the Lower Vertebrates

Oliver B. Cope.

Insects are so diverse in structure and habit and so widely distributed that they come into close contact with all other groups of living things, except the organisms that inhabit the sea. Through the contacts they have become involved in a variety of plant and animal relationships that bear all degrees of complexity and dependency. From one-celled protozoan to man, nearly every animal group has come to develop associations of one kind or another with the insects in the air, on the ground, or in fresh water. Some associations are quite casual. Others are so fixed that the existence of the insect or its neighbor would be gravely threatened by the removal of the other member.

The cold-blooded vertebrates the fishes, amphibians, and reptiles also have diversified habits. Aquatic, semi-aquatic, subterraneous, terrestrial, and arboreal habitats put them close to many insect forms. Many specific relationships between the two animal groups have arisen through the ages. Perhaps no other classes can better illustrate the interrelationships between the insects and their neighbors in nature. It is also appropriate to consider the important economic implications of these associations of the lower vertebrates, especially the fishes.

The cold-blooded vertebrate animals depend on insects for their sustenance in varying degrees. Sometimes, as with the turtles, practically no insect material enters the diet. On the other hand, large groups of fishes could hardly exist without the benefit of abundant insect forage to sustain them. Between these extremes are vertebrate animals toads, small snakes, lizards which although usually not in the habit of eating insects to the exclusion of other kinds of foods, ingest them in great numbers when they are readily available.

Studies by E. M. Uhler, C. Cottam, and T. E. Clarke on Virginia snakes revealed that almost all of 15 species examined had insects in their stomachs. Large insects, such as grasshoppers, cicadas, and caterpillars, were most frequently taken. Smaller forms, such as ants and small beetles, also were found in abundance. Insect remains represented the total contents in some stomachs. In others less than 50 percent of the food was of insect origin. Only a few of the smaller species of snakes, such as green snakes, ground snakes, and ring-necked snakes, are essentially insect eaters; insects contribute moderately to the diets of other species of snakes.

Lizards are the most decidedly insectivorous of the reptile group. Many are vegetarians. Large species specialize in relatively bulky prey, such as crabs and mice, but most small lizards fitted with tongues adapted for capturing insects by lapping them up or by striking at them heavily subsist chiefly on insects. G. F. Knowlton found that beetles, ants, leafhoppers, and other terrestrial insects are staple items in many lizard diets. Red-backed alligator lizards from Santa Rosa Island in California had little else but insects in the digestive tracts. Eighty percent of the stomach contents were darkling beetles. Larder beetles, ground beetles, stink bugs, and plant-lice also were present.

Amphibians the frogs, toads, and salamanders because of their aquatic or semiaquatic habits, come into contact mostly with swimming or flying insects, although adult amphibians frequently seek crawling insects. Swimming or flying insects are readily taken as food by amphibians and in considerable quantities. Salamanders are not highly insectivorous but eat insects readily when they are available. In studies on the tiger salamander, D. S. Farrier found that 74 percent of the stomachs examined had terrestrial arthropods, including ants, beetles, and flies. Thirty-seven percent of the stomachs contained aquatic beetles, flies, and caddisflies. Others have found mayflies, caddisfly larvae, and caterpillars in the food of salamanders.

Frogs are of minor importance as insect feeders, although certain species take insects in abundance. E. W. Jameson, Jr., in studying the western cricket frog, found beetles present as 55 percent of the total food volume, spiders as 24 percent, midge larvae and fly adults as 12 percent, and water boatmen as 7 percent. W. J. Hamilton, Jr., pointed out that the importance of insects in the diet of one species of frog varies with time and place. In summer and fall, when aquatic environments are usually restricted in area, grasshoppers become more important than in the spring. In both young and old frogs, beetles, flies, grasshoppers, and caterpillars were present as 60 percent of the bulk of the stomachs studied.

Toads are more dependent on the insects for nutrition than are other amphibians. The true toads, while omnivorous, taking only living or moving food and endowed with an extremely rapid digestive apparatus, consume large amounts of insect material.

A. H. Kirkland determined that 77 percent of the food taken by Bufo throughout a season was of insect origin, authenticating the reputation of this toad for having a high economic value as a natural suppressor of insect populations. H. J. Pack pointed out that a large population of Bufo woodhousii in Utah in August 1921, was feeding exclusively on sugar-beet web-worms. Each small toad had 24 to 40 worms in its stomach. There was a concentration of toads in the area, and they were eating webworms to the exclusion of their normal diet, a habit that contributed greatly to the control of the undesirable insects. The spade-foot toad, in its immature stages, is an active feeder on aquatic insects, particularly the larvae and pupae of mosquitoes.

THE RELATIONSHIPS between fish and insects as predator and prey are much better known and understood than are those of other cold-blooded vertebrates. Fish living in the ocean do not utilize insects as primary components of their diets, because insects are so poorly represented in the sea. Anadromous fish, which live in fresh water during portions of their life cycles, depend almost wholly on insect organisms during the fresh-water phase. There are fish, also, which inhabit brackish water, gleaning numbers of insects, among other organisms, in their search for food. It is in the true fresh-water haunts, however, that the ultimate in the use of insects as fish food is attained and the debt of freshwater fish to the insects becomes obvious. Both herbivorous and carnivorous insects, either aquatic or terrestrial, are utilized.

The relationships between a population of insects and a particular population of adult fish can be direct or indirect. The fish may capture an aquatic insect in the water of the stream or lake or on the bottom. A flying insect whose habits are terrestrial may be snatched from the air or water surface. Herbivorous insects, such as midge larvae, may be devoured by carnivorous forms, like stonefly nymphs, which in turn may be eaten by fish in streams. Insects may contribute to the diet of a large fish by falling prey to a small fish, which is in turn captured by a larger one. Aquatic insects may be utilized by certain fish during the first part of the fish's development, before dietary habits are changed. For example, the largemouthed black bass ingests quantities of insects until it attains a length of a few inches. After that time, small fish replace insects as the dominant item in the bass diet. All these possibilities are everyday occurrences in the dynamics of food utilization.

STREAM HABITATS used extensively by fish usually have a variety and abundance of aquatic insects. Streams considered suitable for trout and small-mouthed black bass have a good balance of pools and riffles. The bottom topography, aside from providing resting and feeding areas and incubation sites for the fish, makes for the support of a diversity of aquatic insects. Trout in most streams subsist principally on these, whereas terrestrial insects that fall onto the water usually form the second biggest portion of the diet.

The aquatic forms are associated with particular types of stream bottom. In riffles, where shallow water flows rapidly over gravel bottoms, insects with high oxygen requirements dwell the early stages of stoneflies, mayflies, caddisflies, beetles, true flies, and other kinds. In most streams, riffles carry many fold the amount of insect life that is present in pools. Pools, however, provide numbers of fly larvae and mayfly nymphs as fish food, and serve as temporary repositories for terrestrial insects that have fallen onto the water.

The insect types that dominate in stomach samples in stream fish usually follow a pattern, which varies only moderately according to the physical nature of the stream bottom and the chemical nature of the water. Mayflies or caddisflies usually occupy the most prominent place in the diet of trout and salmon. Of lesser importance are true flies, stoneflies, beetles, bugs, and nymphs of dragonflies and damselflies.

Phryganea vestita, a caddisfly.