BIRDS AS ENEMIES OF INJURIOUS INSECTS.

Forest and Bird, Issue 17, 1 April 1929, Page 7

BIRDS AS ENEMIES OF INJURIOUS INSECTS.

Extracted from “The Practical Value of Birds,” Henderson.

In a preceding chapter the work of birds in checking great plagues of insects and rodents has been discussed, but their services in keeping such pests in check at all ordinary times and thus preventing destructive outbreaks are even more important. To fully appreciate such services, especially in connection with injurious insects, three things should be kept clearly in mind: (1) The large amount of food required by birds, in proportion to their size, a matter more fully discussed in another chapter. (2) The large ratio of waste material in insects, which requires a great quantity of them to provide a comparatively small amount of nutriment. (3) The astonishing rate of reproduction among insects, which would soon overpopulate the whole surface of the earth, were it not for the numerous checks upon their increase, none of which can well be spared and among which birds occupy a very prominent position.

The rate of increase of some insects is almost incomprehensible. Murray has estimated that the offspring from a house fly, allowing only 1 out of every 12 eggs to develop, would reach the almost incredible number of 7,600,000,000 during the five months from April 10 to September 10. According to Collinge, if all the eggs of a hop aphis should hatch and all the individuals survive, each female producing its complement, the 12th generation (13 generations per annum) would number 10,000,000,000000,000,000,000 individuals. Obviously a large proportion of them must be destroyed in one way or another, and some species of birds are known to be destroying large numbers of aphids. No agency engaged in keeping these insects in check can be spared. Buckland says that a single pair of potato beetles would produce 60,000,000 offspring in a single season, if unchecked. Certain species of birds feed upon these beetles, and canno't be spared, but the fecundity of the insects and the great increase in the supply of their favourite food render all their natural enemies inadequate, so that artificial means of combating them are necessary. This is true of many other insects. The rate of increase of all destructive insects is so high that, if unchecked by adverse weather and enemies of all kinds, parasitic and predatory, the insects would soon destroy practically all vegetation, unless their very numbers should act as an efficient check upon further increase.

With the fecundity of insects in mind and their consequent potential capacity for destruction, we may now for a moment consider the actual damage they do even with all the checks upon their increase in full operation. Buckland says that the daily ration of a caterpillar is twice its own weight of leaves,

and that if a horse should eat at the same rate he would consume a ton of hay in 24 hours. He quotes Forbush to the effect that a certain flesh-eating larva will consume in 24 hours 200 times its own original weight, which would be equivalent to a human infant eating 1,500 pounds of beef during the first day of its existence. He cites Trouvelot as authority for the statement that the food taken by a single silkworm in 56 days equals 86,000 times its own original weight. What destruction would be wrought by insects were it not for their numerous enemies! It was estimated in 1921 that insects annually destroy one billion dollars’ worth of forest and agricultural products in the United States and that but for the birds the damage would be $444,000,000 more annually- —damage reduced about 28 per cent, by the activity of the birds. Good business sense dictates the reasonable protection of creatures responsible for such a saving. Another estimate of the destructiveness of insects in the United States places it as S2O per year for every man, woman and child in the country, or more than two billion dollars. It is reported that the annual loss caused by insects in this country increased from $300,000,000 in 1863 to $800,000,000 in 1904. and has continued to increase.

According to Forbes, in Illinois the birds destroy about 70 per cent, of the annual insect crop, which surely must act as a very effective check upon their increase. Attwater, in an address to a Farmers’ Congress, declared that the birds in Texas require about 35,000 bushels of insects for food per day, and added that the estimate is too low at that. Think of all the damage those insects and their progeny, coming on in an unending and rapidly increasing procession, would do if permitted to live! Bryant says that the Meadowlarks alone in the Sacramento Valley, California, consume 193 tons of insects daily in feeding their young during the nesting season. Many years ago it was estimated that the annual damage done to the apple crop by the codlin moth in the United States was from $12,000,000 to $15,000,000. At least 36 species of birds are known to feed upon the codling larvae, destroying in some localities from 66 per cent, to 85 per cent, of the insects, according to McAtee, who says: “Since 1746 nearly all entomologists who have published accounts of the codling moth have paid high tribute to its avian enemies, and they are almost unanimous in declaring birds to be the most efficient natural enemies.”

True, with the vast increase in acreage of orchards the birds are not able to control the pests, but nevertheless they render valiant service to the orchardists in destroying a large percentage of the insects. In a badly canker-infested apple orchard in Illinois it was

observed that a large number of species of birds had gathered to feast upon the insects. One year 31 species were seen thus -engaged and in another year 35 species. To ascertain definitely just what food they were taking, 146 of the birds, representing nearly all the species, were shot, and it was found that 35 per cent, of the contents of all their stomachs consisted of canker worms.

On a Maryland farm it was learned by 7 years of observation and investigation, including the examination of 645 stomachs of various species of birds, that 60.41 per cent, of all their food was insects and their allies, and during a sawfly outbreak 7 out of 15 stomachs contained these destructive insects.

Forbes collected a large number of birds in a canker-infested orchard and other specimens of the same species elsewhere and compared the contents of their stomachs. He found that in every instance the caterpillar element of food of those collected in the orchard was, on account of the abundance of canker worms and cutworms, much greater than those collected else-

where.