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RELATION OF BIRDS TO GRAIN APHIDES.
By W. L. MCATEE,
Assistant Biologist, Biological Survey.
Several species of aphides or plant lice habitually feed on growing cereal crops. None of them ever attracted much attention in the United States, however, until the first serious outbreak, in 1890, of an imported species (Toxoptera graminum) now commonly known as the "green bug." Widespread and disastrous irruptions of the green bug occurred also in 1901, 1903, and 1907. In the last-named year the wheat and oat crops of Kansas, Oklahoma, and Texas fell about 50,000,000 bushels short of the average.1
In 1909 grain aphides were excessively abundant and injurious in parts of North Carolina. One badly infested locality near WinstonSalem was visited by the writer for the purpose of learning the relations of birds to the pests. Here the birds in the grainfields were studied daily from March 29 to April 4, and more than 150 stomachs, representing 13 species, were collected for detailed examination. Most of the investigation was made on the farm of Mr. G. W. Hinshaw, to whom acknowledgment of many courtesies is due.
DESCRIPTION OF THE OUTBREAK.
The writer's visit immediately followed a period of hard driving rain, during which the number of aphides was very materially reduced. Some rye plants that had been sheltered by a tobacco barn in course of construction were pointed out as typical of conditions before the rain. These plants bore from 60 to 75 aphides each. Unsheltered plants, however, as was learned by test counts in various parts of the 100 acres of young wheat and rye on the farm, bore on the average not more than one aphis each. An oat field some miles away, which probably was not reached by the heavy rains, was very densely populated with aphides, and as the result of their attacks most of the plants had turned reddish or brown.
1 Bul. 110, U. S. Bur. Entomology, p. 40, 1912.
APHIDES PARTICIPATING IN THE OUTBREAK.
While the green bug was well represented on the grain plants, it was not the most abundant species. That rank was taken by another common and widely distributed species (Macrosiphum granaria). Still another aphis (Siphocoryne avena), often referred to as the European grain louse, was present. While these two species undoubtedly are injurious to growing grain and sometimes destroy parts of fields where they become extremely abundant, it seems well established that their power of destruction is greatly inferior to that of the green bug (Toxoptera). Whether the latter has a toxic effect upon the plants or whether its greater harmfulness is due to some other cause, observations and the experiments thus far performed show that Toxoptera, although smaller than Macrosiphum, is much more destructive to the host plants.1
EXAMINATION OF BIRD STOMACHS.
The discrepancy in economic importance of the aphides concerned in the infestation made it desirable to learn, if possible, the exact numbers of Toxoptera eaten by the birds. Much time was spent in seeking a practicable method of distinguishing the three genera of aphides in the condition in which they were found in the bird stomachs. The attempt was unsuccessful, however, and consultations with specialists revealed the fact that they could not separate all, or even the good specimens, without an entirely disproportionate expenditure of time and effort. Nor is this surprising when we reflect that any of the stomachs might contain a mixture of all stages, from the smallest young to the perfect adults of all three genera. Moreover, plant lice are so extremely fragile that the greater number in almost every stomach were ground up beyond specific recognition. We were forced, therefore, to be content with a simple enumeration of the specimens and the knowledge that probably all of them belonged to one or another of the genera of grain aphides. However, among the adult specimens in more perfect condition all three of the species mentioned above were definitely identified.
While it is unfortunate that we are unable to state in exact terms the relation of birds to the more injurious Toxoptera, we run no risk of mistake as to the economic value of the birds, because all of the aphides are injurious, and birds preying upon them must be given credit for good work.
1 Bul. Univ. Kans., Vol. IX, No. 2, p. 98, 1909.
RECORD OF THE BIRDS BY SPECIES.
The stomachs of three species of birds that contained food yielded no aphides. These species are the chickadee and pine warbler, which, from their arboreal habits, would hardly be expected to feed on grain aphides, and the robin, a bird rather above the size for aphis-eating.
It should be borne in mind throughout that complete enumeration of the plant lice was possible in very few cases. In the majority of instances the finely ground remains of aphides probably represented as many, if not more, individuals than we were able to count among the better preserved material.
GOLDFINCH (Astragalinus tristis).-Of all the species of which a considerable number of stomachs were collected, the goldfinches (fig. 15) made the best use of their opportunities for aphis-eating. Aphides were found in all but 5 stomachs out of a total of 25 collected. No No fewer than 325 plant lice were counted in the contents of one stomach, and the average number of countable aphides in the 20 stomachs was 132.5. They constituted, on the average, 82.75 per cent of the total food. This is a splendid record for this charming little bird, which is popularly known as the wild canary, thistle
bird, or lettuce bird. A flock of goldfinches frequented the vicinity of a telephone line running through the farm, and from their perches on the wires the birds were continually flying down among the rye for aphides.
PINE SISKIN (Spinus pinus).-Only one pine siskin was collected. It had eaten more than 80 aphides, which composed practically the entire stomach contents.
VESPER SPARROW (Poacetes gramineus).-This was the most abundant species of regular occurrence on the farin. Twenty-two stomachs were collected, 15 of which contained plant lice. The average percentage of the food composed of aphides was 19.5, and the largest number counted in any stomach was 42. Manifestly the vesper
sparrow was not so fond of plant lice as most of the other sparrows. It ate about as large a proportion of beetles as of aphides..
SAVANNA SPARROW (Passerculus sandwichensis savanna).-Thirteen out of 20 savanna sparrows collected had eaten aphides. The largest number secured by any one bird was 130 and the average num
ber 63.5 Aphides
(fig. 16) was common and proved to be a good aphisconsumer. Thirtyfive birds out of 48 collected had eaten plant lice. The largest number taken by any individual was 260; the
average number 94.76. A little more than 45 per cent of the total food of these birds consisted of grain aphides.
(Spizella pusilla).— 5 Only 6 field spar
rows were collected. Three of them had
eaten plant lice to the average extent of 96 per cent of
their food. The numbers of aphides that could be counted in their stomach contents were 87, 180, and 196, an average of 154. This is a praiseworthy showing, and it would be interesting to know whether it would have been maintained had a larger number of specimens been examined.
SNOWBIRD (Junco hyemalis).-Only 7 snowbird stomachs out of a total of 17 contained aphides. The plant lice could be counted in only one instance, 14 being distinguished. The average percentage of the food composed of aphides was only 5.2, quite the lowest record of any of the aphis-eating birds.
SONG SPARROW (Melospiza melodia).-Two song sparrows (fig. 17) were collected, of which one had eaten about 50 plant lice, which composed 80 per cent of its food.
TITLARK (Anthus rubescens).-A very large flock of titlarks visited the farm one day during the investigation. Only one could be collected, but it had eaten 100 or more aphides, which constituted about 70 per cent of
most of the time among the aphis-infested grain. The results are as follows: Goldfinch, 300 individuals; vesper sparrow, 2,590; savanna sparrow, 70; chipping sparrow, 245; field sparrow, 20; snowbird, 70; and song sparrow, 6.
The number of song sparrows, and probably also of field sparrows, is not above the normal for the nesting season, and hence is at the minimum for the year. All of the others were far more abundant than they would be in the breeding season; in fact, two species, the vesper sparrow and snowbird, do not breed in the vicinity of Winston-Salem. The period of observation was in the height of the migration of such species as the chipping, vesper, and savanna sparrows. The activity of migration at the time is further evidenced by the occurrence on one day each of a flock of 100 pine siskins and one of about 5,000 titlarks.
NUMBER OF APHIDES EATEN BY THE BIRDS.
In estimating the quantity of food consumed by birds feeding on a mixed diet of average digestibility we usually regard the day's