As a result of the rapid multiplication of the aphides, the infested clusters were soon completely overrun by them. The species avenæ reached the height of its abundance earlier than either sorbi or pomi, and was likewise the first to show retrogression in numbers after reaching the point of maximum reproduction. Early in June the insects abandoned the fruit clusters and, in their efforts to migrate, collected on the netting, where they died. By late June the force of the attack by sorbi had spent itself, while pomi remained on the clusters thruout the summer, altho the numbers were decreased during midsummer.

COMPARISON OF INJURIES BY THE DIFFERENT APHIDES.

The most severe injury was shown with the early infested clusters attacked by sorbi and those infested with pomi. Clusters one to six, inclusive, of the sorbi series and the same lots of the pomi series were so heavily coated with the aphides that they soon became a sticky, sooty mass; and the fruits were completely destroyed.

The attack by avena, while of shorter duration, was apparently no less intense while it lasted; for the insects clustered on practically the entire surfaces of the infested clusters, the leaves, stems, developing fruits, and even the petals being overrun with them. Nevertheless, the injurious effects of the feeding by this species were much less obvious than with the other species. Injury by avena was evidenced by a gentle curling of the leaves, and in some cases by a slightly one-sided growth of the fruits. At an early date many of the affected fruits showed a distinct stippling with minute black points and there was also a brownish discoloration on the upper surfaces, which may have been due to some other cause and is as yet not clearly understood.

By the last of May the clusters infested with sorbi showed considerable curling of the foliage; and at this time, even before all of the petals had dropped, there was in some cases a marked curling of the fruit stems and contortion of the young fruits. The curling of the foliage became serious early in June, and by this time many of the fruits and fruit stems had become badly distorted. At this date pomi had not caused such noticeable injury to the fruits. As the infestation continued the damage grew more striking, and on June 22 most of the fruits of the sorbi and pomi series were badly injured. A number of the clusters of the sorbi lot were so seriously

affected that the fruit was entirely destroyed. The contortion of the fruits, due to sorbi, frequently took the form of a reduction in growth on one side, resulting in a curving of the axial diameter, frequently accompanied by more or less protrusion of the calyx, with a star-shaped series of prominent ridges radiating from the center of the calyx end. Contortion caused by pomi was quite similar, but seemed to be characterized by a greater tendency for the entire fruit to become angulate, with longitudinal elevations and depressions.

At the time of picking of the fruit, the apples of the avenæ series showed no contortion, altho in one case the fruit was slightly misshaped. Most of these fruits were poorly colored, showing a yellowish-green hue. Several displayed a brownish discoloration on one side, which may have been due to some other factor, while nearly all of the apples were more or less stippled with minute black points.

The majority of the fruits of the pomi and sorbi series which had undergone extreme contortion dropped before the fruit was picked. The one fruit of the pomi lot was about normal in shape, but was rather pale in color, and was slightly stippled with the fine black points, as has been previously noted. The apples of the sorbi series, at the time of picking, while not showing the most extreme type of distortion which is sometimes found, were more generally ill-shaped than those of the other series. The injury by this species was usually indicated by the protuberance of the calyx end with a star-shaped series of elevations radiating from the calyx cup. In less severe cases the calyx basin and the depression around the stem seemed abnormally shallow and flat bottomed. As was the case in the other series subject to attacks by aphides, these fruits were poorly colored and frequently stippled with fine black dots.

EFFECTS OF ATTACKS ON SETTING OF APPLES.

"

As was brought out in the discussion of last year's studies, the attack of sorbi was found to increase the average number of fruits per cluster, resulting in the clusters of so-called "aphis apples which are frequently seen in infested orchards. An examination of Table I will show that the severe and early infestation of the first six clusters resulted in the dropping of the fruits. This was apparently a destruction of the young fruit, however, rather than a failure

1

of the fruit to set. In the case of clusters Nos. 7, 8, 10, 11 the infestation seems to have become severe before the normal number of fruits of the cluster had dropped, and yet the attacks were not so injurious as completely to destroy the fruits. As a consequence the normal reduction of the numbers of the fruits commonly known as the "June drop" did not occur, with the result that there were from three to five rather small apples to each cluster. Later infestation did not prevent the usual reduction in numbers of the fruits, and smaller clusters therefore resulted, which in some measure resisted the effects of the aphides as far as premature dropping was concerned. Of the clusters infested with pomi there was only one in which the fruits were not completely destroyed, and in this instance the cluster contained only two apples. It seems quite probable, however, that had the early infestation been less rigorous, larger numbers of apples per cluster would have remained in this lot. The average number of apples per cluster was also somewhat raised in the avenæ series. However, it appears that there was less tendency to form large clusters in this series than was the case with the sorbi series.

EFFECTS OF ATTACKS ON GROWTH OF APPLES.

The attacks of all species of the aphides included in this study resulted in a diminution of the size of the fruits that were infested. The greatest inhibition in growth occurred in the series subject to attack by sorbi, which is graphically compared with the growth of the check fruits in Fig. 8. It will be noted that early in the season the axial diameters were greater than the transverse measurements. The transverse diameters increased more rapidly, however, and soon became greater than the axial diameters. In the check series the two diameters were approximately equal on June 30, while the sorbi series were equal about July 12. In other words, the aphis attack inhibited the growth of the transverse diameter to a greater extent proportionally than that of the axial diameter. This was found to be true in the case of each species and varies directly with the amount of injury caused by the species making the attack.

Similar curves of growth for the apples of the series affected by avenæ and pomi would occupy positions intermediate between the curves here shown.

JUNE 30

JULY 15

Figure 9 represents approximately the average monthly sizes of the apples of the several series. It will be seen that the fruits of the sorbi lot suffered the greatest reduction in dimensions. Those

SOUND APples.

1, Sorbi series; 2, pomi series; 3, avenæ series; and 4, checks. White
bars, axial diameter; black bars, transverse diameter.

of the avenæ series closely approach the checks in size, while the pomi series are slightly smaller. These measurements should scarcely be regarded as typical of the attacks of pomi, for an examination of