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The poplar and willow borer is of greater economic importance than has generally been supposed. It is a serious pest of practically all species of willows, and where the beetle is abundant the damage done is extensive. This is especially true of all varieties of ornamental willows. Also the production of basket willows is greatly reduced and in many cases stopped by the work of this insect. To many species of poplars it is very injurious, especially when the trees are young (figs. 111 and 112). In many nurseries the production of poplar stock has been discontinued owing to the prevalence and injuriousness of the pest.
The willows (Salix species) and the poplars (Populus species) are regarded generally as of not much value. However, many willows are used extensively in landscape work, not only for their quality of rapid growth but also for their beauty. Species native to this country line the streams and encroach on the boundaries of lakes and ponds, serving a very useful purpose as holders of the soil. The carolina poplar, though not looked upon with much favor as a shade tree, has been and is being planted rather extensively in recently developed areas. This is especially true in the Middle West, where the trees soon become suitable for lumber. Extensive plantings made from forty to fifty years ago in the Middle West, and also the great areas of cottonwood in the lower Mississipp Valley, are now being lumbered. The product finds a ready sale as lumber and commands a good price for excelsior. The carolina poplar is used also on sandy areas to keep the soil from being washed away.
The balm-of-Gilead poplar, which is used generally as a shade tree, is severely attacked by the borer. The branches serve excellently for the development of the insect, and in sections where it is prevalent scarcely a sound tree can be found. The branches, weakened by the larval burrows, are broken off by high winds and ice storms, rendering the trees unsightly.
In Europe the beetle has proved a pest to many species of willows, poplars, and alders. Many accounts are given of its destructive work, and there is no doubt that it is proving even a worse pest in America.
The beetle, recorded as having first appeared in America in 1882 (Juelich, 1887), has not spread very rapidly. It is at present known to occur from Maine west to Ontario and North Dakota and south to the District of Columbia. Throughout this area it is restricted to certain localities, and once introduced it does not spread rapidly unless carried by some agency. It is reported by Fletcher and Gibson (1909) as occurring at Dundurn,
Saskatchewan, but nothing is known as to its spread in this province. It has not been found west of the Great Plains.
The preceding statement as to the general distribution of the beetle is based on letters received from various officials regarding the situation in their respective States. P. A. Glenn informs the writer that in Illinois the beetle is abundant about Chicago and occurs generally in the northern fourth of the State, while in the central and southern parts it has not been found. Dr. Fracker reports it as widespread in the nurseries of Wisconsin, where it has been found as far north as the shores of Lake Superior. Blatchley and Leng (1916) state that it has not yet been found in Indiana. Professor R. H. Pettit says that it is common in Michigan and probably occurs wherever the carolina poplar grows. In Ohio it seems not to have spread to any considerable extent since its first appearance there in 1901. It has not yet been found in Nebraska, South Dakota, Iowa, or Indiana.
In Canada, according to Caesar (1916), it is well distributed throughout Ontario and is also recorded from a few localities in the province of Quebec.
LIFE HISTORY Although this beetle has been a serious pest in Europe for hundreds of years, its life history has never been fully investigated by European workers. Even at present there is the widest divergence between the accounts given by American and by European writers. This is brought out in detail in the discussion of the various activities of the different stages.
The adult The poplar and willow borer (Plate xxII) belongs to the great group of snout beetles, Rhynchophora, and to the family Curculionidæ. This family contains an immense number of species, many of them very serious pests, including the common plum and quince curculios. The beetle measures from 1 to 3 inch in length, is robust, and is elongate-oval in shape. It is densely clothed with black and pale-colored scales, intermixed with erect, large, black bristles. The pale scales cover the apical third of the elytra and form an irregular band on the basal third; the underside of the prothorax and part of the legs are also densely clothed with them, and the remainder of the body bears a few scattering ones. The beak is curved, is as long as the head and the thorax, and lies when at rest almost completely concealed in a groove on the ventral surface of the thorax. The antennæ are elbowed and reddish brown, with an unsegmented club.
The beetles begin to appear during the latter half of July, becoming abundant in August. The writer found them present on poplar trees
as late as October 7. After that date they could not be found on the trees in the writer's experimental plot. They are sluggish, very inactive insects, and move with a slow, lumbering gait. When they are disturbed, either by being jarred or by any sudden noise, they do not fly but feign death and drop to the ground, the beak and the legs being closely drawn against the body. They remain quiescent usually for a minute or two before attempting to crawl away. When handled they emit a squeaking noise, produced by the rubbing together of parts of the body. Though close watch for it has been kept, flight has not been observed, and no one has recorded the beetles as spreading by means of flight. Whether or not they are incapable of flight the writer has not been able to determine.
Shortly after the beetles emerge from the pupal cells they begin to feed, selecting young, tender shoots. The bark is punctured by the beak, a round hole being formed down to the cambium layer, on which the beetles largely feed. The beetles are voracious feeders, and when they are abundant the young one-year-old shoots may be so completely riddled by the feeding punctures that they shrivel and die. So far as the writer's observations go, the beetles do not feed on old bark, but confine themselves to the young and succulent twigs. Punctures in old bark are for the deposition of eggs, and these always appear some weeks after the beetles have been feeding.
The beetles do not seem to migrate to any considerable distance. Although the wings are perfect and apparently suitable for flying, yet the beetles have never been observed in flight or attempting to fly. In the nursery it is not uncommon to find one block badly infested, whereas a block somewhat distant may be only slightly injured. Change of location in the growing of poplars from year to year frequently makes a marked difference in the degree of injury. One block of about 15,000 trees in a large nursery near Geneva had an infestation of nearly 50 per cent in 1915. A block of about the same number of trees situated threefourths of a mile distant showed in 1916 only a small infestation, 3-5 per cent, in the check rows. The beetles were abundant in 1915 in the block ready to be dug, and apparently they had confined their egg-laying operations to the poplars from which they had emerged. As this is true in all the cases coming under the writer's observation, it can readily be seen that a block ready to be dug, showing only a small percentage of infestation, may make an ideal center for distribution. As the nurseryman discards only severely injured stock, a block with such a low percentage of injury will practically have all the trees fit for sale, and in this way every egg deposited will be shipped away to start new infestations, When trees show considerable injury they are discarded and burned (fig. 113).
Although the beetles do not fly, yet they are undoubtedly well able to walk considerable distances. How far has not been determined, but they have been found a goodly distance from any of their food plants resting quietly on the trunks of various trees. This is especially true in the spring.
FIG. 113. A PILE OF DISCARDED CAROLINA POPLARS IN A NURSERY Mating does not occur until ten days or more after emergence. This period is spent largely in feeding, and the beetles do considerable damage at this time to the vigorous growing shoots. Copulation occurs freely and a pair may remain several days in copula. Not only does mating last a considerable time, but it may be repeated again and again at different times.
Egg laying Shortly after copulation the females seek out suitable places for the deposition of their eggs. They choose branches or parts of the trees more than a year old, and deposit their eggs in the corky parts of the bark. The eggs have never been found in one-year-old stock, but only in wood two years old or older. Favorite places for egg laying are lenticels (figs. 114 and 115), scars, bases of branches, injured areas, or about the base of buds where the bark is somewhat thick. With her beak closely applied to such an area the female beetle at once begins to eat into the bark. FIG. 114. EGG PUNCTURES AT THE Gradually she deepens the round hole
SIDES OF LENTICELS until her entire beak is buried, up to the eyes. The time required for this operation varies from a few minutes to thirty or forty minutes. At the bottom of the hole the beetle may round out two or three small lateral cavities, or she may be content with only one. In the majority of
cases she does not dig out extra cavities, but uses the hole made for the deposition of but a single egg. When the cavity appears satisfactory, the beetle inserts her ovipositor and deposits from one to three or four eggs, depending on the kind of cavity dug. Then, reversing her position, she closely packs the eggs, both with beak and with antennæ, and covers them with fine pieces of the wood.
Egg laying continues from early August until October, but the number of eggs
single female has not been ascertained. Whether or not, under New York conditions, all the females deposit all their eggs during this period, has not been finally determined. All American workers report that egg laying is finished in the autumn, and that the beetles do not hibernate but evidently die after the process is completed. In the
writer's work no hibernating adults were found in the spring in the nurseries, though it should be understood that no extended search was made for them. In the writer's experimental plot, adults which evidently had hibernated were taken on April 21, May 1, and June 6, 1916. One was fresh and clean, apparently having but recently emerged from its pupal chamber. Though these were observed, they may be only rare occurrences rather than represent a normal mode of hibernation. Furthermore, egg laying was not observed in the spring,
and in the treated plots there was no FIG. 115. EGG IN SITU, WITH OUTER evidence that any eggs were laid after
application of the various treatments. Had egg laying in the spring been normal, certainly the treated plots would not have shown such a high percentage of control.
The egg when laid is pure white in color but it becomes pale cream when a few days old. The shell is very thin and fragile, somewhat viscous, and without any distinctive markings. The egg is oval in outline, measuring 1.1 by 0.8 millimeters. The shape varies considerably, since owing to the softness of the shell it is easily modified by the shape of the cavity in which the egg is laid.
The egg stage lasts from eighteen to about twenty-five days, depending largely on weather conditions. The first observation of eggs hatching out of doors was on October 2, 1916. Undoubtedly many had hatched