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PLACE OF EGG DEPOSITION.

Under natural conditions the majority of the eggs are laid in the ground at the bases of the trees upon which the insects are feeding. Most of them are placed in the soil just beneath the surface, altho a few are found to a depth of one inch, and some are deposited on top of the ground. They are often deposited loosely, but generally are attached by a gluish substance to rootlets, small stones and rubbish. It is therefore often difficult to remove the eggs without injuring them. In the breeding cages it was necessary to remove the eggs as they were counted, and as a result many were injured so that only a small percentage hatched.

During the course of our observations eggs were generally found glued to the bases of the trees from a depth of about an inch beneath the soil to a height of several inches above the surface. An exception to this habit was observed in a cherry orchard at Westfield where eggs were detected over all portions of the trunks as high as the forks of the limbs, or as much as three feet above the ground.

Moisture appears to be the controlling factor in the selection of a suitable place for oviposition by the females and so during periods of ordinary weather the females place the eggs in the damp soil. However, during rainy periods the bases of the trees are damp and are apparently attractive to the beetles for purposes of egg laying. In the cherry orchard at Westfield the trees were headed low so that the trunks were well shaded and remained moist for a considerable period. Besides, rainy and cloudy weather prevailed from June 14 to June 21, during which time these trees were continuously wet. By reason of these conditions beetles apparently were induced to lay eggs in what appeared to be unnatural situations.

PROTECTION OF EGGS.

The deposition of the eggs in the soil would naturally protect them from many predaceous enemies, but the chief protection is doubtless against unfavorable physical conditions, such as dryness and heat from the direct rays of the sun. It is believed that the latter is the more important factor because of the fact that no destruction of the eggs, by either predaceous or parasitic enemies, was noted, even during 1915 when eggs were deposited in such abundance.

DURATION OF LIFE OF THE CHERRY LEAF-BEETLE.

The first larvæ of the cherry leaf-beetle were found on July 1, 1915, but the maximum number emerged during July and August. During 1916 the first larvæ appeared on June 23, while the majority of individuals hatched during July and the first half of August. In the cages a number of the hibernated beetles died during June but July was the month during which most of the beetles succumbed. The last female died August 13, but four males lived until the last week of September. The average life of the beetles therefore was about twelve and one-half months. Of the specimens displaying exceptional longevity, the greatest duration of life of the female was not over fourteen months, while the males may have survived for about fifteen months, unless they hatched later than the average during the preceding year, in which case their period of exist ence would be somewhat less than that mentioned.

1

THE EGG.

DURATION OF INCUBATION PERIOD.

In his observations of the species at Washington, D. C., during 1899 Chittenden 1 states that the first egg hatched on June 26, which was eleven days from the date of deposition. During July, 1915, Herrick 2 and Matheson at Ithaca obtained as the longest incubation period eighteen days, and the shortest period was fourteen days, with an average of sixteen days.

Owing to the difficulty of securing eggs from the beetles in the observation cages during 1915, it was found necessary in order to secure data on the length of the incubation period to remove eggs from the soil at the bases of certain trees, precautions being taken to obtain them shortly after they were deposited. These were transferred by means of a camel hair brush to a cage consisting of a Van Tieghem cell cemented with vaseline to an ordinary glass slide. On the bottom of the cell a few fibers of cotton were spread to hold the eggs and prevent their rolling; and a drop of water was placed in the cell occasionally to keep the air moist, reproducing as far as possible the conditions of moisture of the earth from which the eggs were obtained. A cover glass was used to enclose the cell.

1 U. S. Dept. Agr. Div. Ent. Bul. 19, n. s., p. 93. 1899.

2 Jour. Agr. Research, 5:946–947. 1916.

Three lots were started with eggs which were laid on June 17 and one lot with eggs which were deposited on June 18, 1915. The records of hatching are summarized in Table VII.

TABLE VII.- INCUBATION OF EGGS OF CHERRY LEAF-BEETLE AT FREDONIA, N. Y.

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Record of eggs deposited on June 17, 1915.
(Temperature records in degrees F.)

Duration of incuba

tion period (days) 10 11 12 13 14 16 18 19 20 21 Average daily mean

temperature to

day indicated.... 65.9 65.6 65.6 66.2 66.3 66.6 66.5 66.4 66.1 66.1

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21 days.

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10 days.

15.8 days.

66.3 degrees F.

By reason of the experience during the previous year it was possible to carry on more extensive observations on the egg stage during 1916. In the breeding cells eggs hatched over a period ranging from June 23 to August 1. These dates do not represent the entire period of hatching of eggs under normal conditions, which was somewhat greater; and they are of value only in connection with the particular eggs which were under observation. Notwithstanding the considerable number of larvæ that hatched, it should be stated there was high mortality among the eggs, which was largely ascribed to the methods of handling them. The records dealing with the incubation of three hundred thirty-nine eggs are given in Table VIII.

TABLE VIII.— INCUBATION OF EGGS OF CHERRY LEAF-BEETLE AT Fredonia, N. Y.,

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Average daily mean temperature (based on hourly temperatures) for period for all eggs.

23

9

13.20.13

26.96.75

74.5

Number of eggs hatched.

339

Coefficient of correlation between mean temperature and length of incubation period...

-.705.018

RELATION OF TEMPERATURE TO THE LENGTH OF THE INCUBATION PERIOD.

A comparison of Table VII and Table VIII shows that the daily mean temperature during the incubation period was lower during 1915 than during 1916 and that the incubation period for 1916 averaged 2.6 days less than during the preceding year. In order to determine the relationship that exists between the daily mean temperature and the incubation stage of the cherry leaf-beetle, we have calculated the coefficient of correlation between these two phenomena using the data of 1916. This coefficient was found to

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be-.705.018 and is rather high, showing there is marked correlation between the two phenomena. The coefficient is negative, which means that as the temperature increases the length of the egg stage decreases. From the regression equations we learn that on the average an increase of one degree in the daily mean temperature of the incubation period decreased the period by practically one day in our cages.

While the above analysis shows the important part temperature plays in determining the incubation period, it should also be noted that it is not the only factor to be considered. As has been observed with other species of insects, there existed marked variation in the length of the period of incubation in the same cage where the eggs were of the same age and presumably were subjected to identical conditions. Examples of this variation may be noted in lots 1, 5, 7 and 8 of Table VIII.

HATCHING.

Time of hatching of first, maximum number and last eggs.-The period during which hatching occurs extends from the latter part of June until the latter part of August of a normal year. The incubation of the eggs is influenced by the weather as noted above and during abnormal seasons variations occur in the length of the hatching period. It appears from the data that the maximum time during which hatching takes place is two months.

During 1915 the first larvæ were found in their native haunts at Fredonia on July 1, and Cushman1 and Isley record larvæ hatching in their cages as late as August 18. These facts would give seven weeks as the period during which hatching occurred during this season.

During 1916 the first larvæ of the season were found both in the field and in breeding cages on June 23, but the eggs hatched very slowly until after July 4. The maximum number of larvæ emerged from July 12 to 31. With the eggs under confinement the hatching period ended about August 20, and during the months of July and August the weather was quite warm. The hatching period extended slightly over eight weeks, during the first two and last three weeks of which the relative number of larvæ emerging was rather small. 1 Loc. cit. p. 17.

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