THE LARVA.

ACTIVITY OF LARVA AFTER HATCHING AND NATURE OF FEEDING.

After emerging from the egg the larvæ ascend the tree at the base of which the eggs were laid and begin feeding on the foliage. If the tree happens to be a bird cherry the insects are able to develop to full growth, but if the eggs are placed at the base of another species of cherry the larvæ feed for a short time and if unable to find foliage of the bird cherry they have, according to our observations, invariably succumbed before passing thru the last instar, most of them dying during the first instar. The larvæ are active creatures, capable of crawling rapidly, and they feed almost entirely on the underside of the foliage. They eat the lower epidermis and the parenchyma, but generally do not eat thru the palisade layer and the upper epidermis. Occasional feeding occurs on the upper surface of the leaf. The nature of their feeding is shown in Plate XL. The injured portions of the leaves turn brown and portions of the palisade layer and the upper epidermis die and drop out, thus causing the foliage to have a ragged appearance.

FOOD PLANTS OF LARVA.

At the time of the outbreak of the cherry beetle during 1915 attempts were made to rear larvæ on foliage of the choke cherry, rum cherry and cultivated sour cherries and peaches, but in no instance was it possible to rear them beyond the second instar, and the majority of the creatures died during the first instar. These experiments were repeated in 1916 with identical results. Cushman1 and Isley, on the basis of their studies during 1915, reported similar experiences. In the cages we were able to rear the larvæ only on the bird cherry, and in all field observations the only foliage on which the larvæ have been observed to reach maturity was that of this plant. It is for this reason that P. pennsylvanica has previously been designated as an essential food plant of this insect. It appears that infestation of cultivated cherries and peaches has its origin in the bird cherry. Observations indicate that where these fruits are grown outside the range of the bird cherry they are seldom attacked, and the probability of injury by the insects decreases 1 Loc. cit. p. 2.

directly as the distance from the range of the bird cherry increases. These facts also seem to warrant the statement that cherry and peach growers need pay no attention to any stage of the pest except the adult. The intermittent character of the outbreaks may be explained by the fact that only during periods of abnormal numbers of adults, especially if food be scarce, do these insects migrate to cultivated trees.

GROWTH OF LARVA.

The larvæ feed voraciously and grow rapidly, especially during the warm periods. Two molts occur during the feeding period and a third molt takes place upon transformation to the pupal stage, which make three larval instars for this species.

Molting. In molting the skin splits along the median line of the back, usually extending from the occiput down the thorax over several segments of the abdomen.

The larva first withdraws its head and thorax and after securing a hold on the leaf by means of its feet gradually withdraws the abdomen. During a portion of this process the larva was motionless and this quiescent period was followed by writhing motions to withdraw portions of the body from the skin. These operations were repeated at intervals of several minutes. The exact length of time required to molt has not been noted but on July 23 a larva was found in the course of molting at 3:20 P. M., at which time the head and body were pea green in color, the head being slightly lighter in shade. The spiracles were black and the legs were the same color as the head. At this time the larva had the shed skin on the abdomen from the eighth to the anal segment and it was motionless. The specimen was then closely observed and notes were made of any changes in its appearance as follows:

3:25 P. M. Head of the same color as at 3:20 P. M., body slightly darker, feet intermediate in color between that of head and body. A few writhing movements of the body, head raised several times at short intervals, then motionless again. Head measured .54 mm. in diameter which shows that the larva was in the second instar. 3:30 P. M. Color of head as before, body slightly darker, feet almost same color as body, first appearance of the formation of spots, larva generally quiet.

3:35 P. M. Head darker, body becoming an olive green. The larva was active at longer intervals than in preceding periods.

3:37 P. M. Larva leaves molted skin. Head a light olive green, body becoming quite dark, all the darker areas a dark olive green.

3:40 P. M. Head exhibiting same color as at 3:35 P. M. Traces of black begininng to show in the spots. Anal plate not as dark as the spots.

3:45 P. M. All portions of the body growing gradually darker. Spots prominent.

4:00 P. M. Head of a dark olive green with traces of black. Spots and anal plate very dark but not fully colored, legs slightly lighter than the body, claws and tarsi almost black. Larva resting quietly.

4:05 P. M.

Larva crawling.

Body and legs almost black, head slightly lighter than the body.

4:15 P. M. Body and legs black, head a dark brown.

These notes show that the act of molting may consume at least twenty minutes and that about an hour is required for the full development of the color of the larva.

Increase in the size of larva.- In order to measure the rate of growth, individual larvæ were placed in separate cages shortly after hatching and these were carefully observed until they entered the soil to pupate, daily measurements being made of the head diameter and length of each individual. Fifteen larvæ under observation passed thru the three instars and finally entered the soil. The measurements of these were made under a compound microscope with a lens arrangement whereby one space of the micrometer was equal to .0325 mm.

A leaf upon which the larva were resting was placed under the objective and the mirror set to reflect light thru the leaf, thus making it easier to note the position of the image on the micrometer stage. Owing to the size of the larvæ no measurement could be carried to a greater degree of refinement than the space of one division on the micrometer.

The diameter of the head of the larva of the cherry leaf-beetle, as is true of other species, does not increase during an instar, but the change in size occurs at the time of molting, while the epidermis is capable of expanding and before the chitin has become hardened. The average diameter of the head for the several instars was found to be as follows: First instar, .38 mm.; second instar, .54 mm.; third instar .76 mm. There was some variation in the sizes of the head in the different larvæ, as follows: First instar, from .36 to .39 mm.; second instar, .52-.57 mm., and third instar, .72-.78 mm.

The length of the body varies with the development of the larva, growth being constant between molts. There is no doubt considerable difference in the sizes of different individuals during each period of growth, but as it would have required a larger number of measurements to ascertain the extent of variation than the time at our disposal would allow, no effort was made to determine it. The variation in length of larvæ for each instar for a goodly number of specimens is as follows: First instar from 1.6 to 3.3 mm.; second instar, 2.3-4.5 mm., and third instar, 3.6-7.2 mm.

DURATION OF THE SEVERAL INSTARS AND THE LARVAL STAGE.

Observations of the length of the several instars were made on fifteen larvæ over a period ranging from July 13 to August 7. The data are given in Table IX.

TABLE IX.-STAGES OF DEVELOPMENT OF CHERRY LEAF-BEETLE FROM LARVA to ADULT. FREDONIA, N. Y., 1916.

Average for different larval instars: 1st, 4.3 days; 2d, 3.2 days; 3d, 3.2 days. Period in soil 10.7 days.

As the temperature during the months of July and August of 1916 was considerably above normal, attention is called to the studies on the different larval instars by Herrick1 and Matheson at Ithaca and by Cushman 2 and Isley at North East, Pa. The observations were made in 1915 during the months of July, August and September, which were cool and wet, with temperatures below normal. The data dealing with each instar are considered separately.

First instar. In Table X there are summarized the more important facts by the foregoing observers, as well as our own, which bear on the duration of the first instar.

TABLE X.- LENGTH OF THE FIRST INSTAR OF THE LARVA OF THE
CHERRY LEAF-BEETLE.

Author

3

4.3

July 23-30, 1915 | Aug. 11-24, 1915 | July 13–29, 1916

It will be noted that the longest period required for a larva to pass the first instar was eleven days and the shortest period three days. The average length of time for this stage as based on the records of two years in the Lake Erie valley is 4.6 days. It is unsafe to make a general average of the foregoing three sets of observations, owing to the fact that the climate of Ithaca is different from that of the Lake Erie valley, while the climate of North East, Pa., and Fredonia are quite alike, both being situated near Lake Erie and only thirty miles apart. As Ithaca is located in the plateau region of southwestern New York its climate resembles more closely that of the southern tier of counties than does that of Fredonia, wherefore the data from the former should be fairly representative of the native habitat of the insect.

Second instar. The data dealing with this instar are summarized in Table XI.

TABLE XI.— LENGTH OF THE SECOND INSTAR OF THE LArva of the
CHERRY LEAF-BEETLE.

Minimum length (days)
Average length (days).

7

3

3.6

5

2

3.2

Period of observation.. July 27-Aug. 4, 1915 Aug. 5-29, 1915 July 17-Aug. 3, 1916

The longest period for the second instar was seven days, the shortest two days and the average for two years in the Lake Erie valley was 3.4 days.

Third instar.— The data on this instar are tabulated in Table XII.

TABLE XII.- LENGTH OF THE THIRD INSTAR OF THE LARVA OF THE