By Fred E. Brooks
Associate Entomologist U. S. Department of Agriculture
The prevalence of insect pests need not be regarded as an alarming obstacle to nut growing in the North, and yet there are numerous species of insects which are capable of destroying our nut crops. On the whole I presume there are fewer insects that attack nuts in this country than commonly attack apples, but apple growers are not limited in planting nor prevented from making profits on account of insect depredations. Neither should the probability of more or less insect injury discourage the would-be planter of nut trees.
The presence of an insect in any locality may mean, among other considerations, that the soil, and climatic conditions of that locality are favorable to the plant upon which the insect feeds. We may be sure that wherever the Baltimore butterfly is abundant, nearby is a congenial spot where the turtle's-head, the food plant of the butterfly, flourishes. Just so, in localities where there are many chestnut weevils we may expect to find chestnut trees thriving and fruiting generously. The same is true of the associations of many other insects and plants.
Theoretically speaking, one would not care to risk the expenditure of much time or money in propagating a plant in a region that was destitute of insects that might attack that plant. The absence of such insects would possibly indicate a lack of natural conditions favoring the growth of the plant in question. Thus the presence in any locality of insects that feed on nuts may mean that nuts thrive naturally in that locality and that insects are there because of the abundance of a favorite food.
May I hasten to add, however, that this fact should not lead to an under-estimation of the possibilities of insect destructiveness, nor encourage lax methods in dealing with injurious species. In the beginning of any nut-growing enterprise we should anticipate the coming of insect pests and be ready to meet them. The planting of pure stands of native nut trees sets up a condition under which insects coming from the forest may increase more safely and rapidly than under the more hazardous environment of a scattered forest growth. This applies to cultivated plants generally. It is true of an orange grove, a cornfield or a potato patch. The mass planting of any crop is quite sure to call sooner or later for measures to offset the stimulus which such plantings offer to insect increase.
Reference may be made to a familiar nut plantation which illustrates a natural result of neglecting one of the insect factors. This plantation is the government's chestnut orchard at Bell, Maryland, which was planted for scientific purpose some years ago by Dr. Van Fleet. This orchard of around one thousand trees contains numerous species and varieties of chestnut, some of which bear fruit every year. The various scientific projects carried on in this orchard in the past have all been of such a nature that they called for no consideration of weevil increase. Many nuts have been allowed to lie under the trees until the weevil larvae issued and entered the soil. This has resulted in a constant increase of weevils until infestation of the nuts became practically one-hundred per cent. All nuts of the crop of 1922 were so wormy that when planted they failed to germinate. Injury to the crop of 1923 seemed somewhat less severe, but its extent may be indicated by the fact that 3080 nuts from this orchard which were kept by the speaker in rearing jars yielded 11,085 worms. In the woods adjacent to the orchard the native chestnut trees are disappearing on account of the blight, and presumably weevils are on the decrease. Within the small area of the orchard, however, the increase has been abnormal, due, as has been indicated, to the peculiarly favorable and man-made conditions. If, from the time the trees of the orchard began to bear, the investigations being carried on had called for close gathering of the nuts at maturity and the destruction of all the worms that issued from them, there is little doubt that infestation would have been kept within reasonable bounds. At present, after two years of attention to the collection of ripening nuts, there is an apparent decrease in the number of weevils. Strong emphasis should be placed upon the importance of gathering chestnuts as soon as they are ripe and prevention of the worms from reaching the soil. This is especially true of districts where woods surrounding chestnut orchards do not contain bearing native chestnut trees.
The Nut Weevils
Now that the subject of nut weevils has been introduced, let us consider in more detail these grotesque, long-snouted insects whose larvae, or grubs, play havoc with so many of our nuts. Most of us have had the experience of gathering in autumn rich stores of our delicious native chestnuts. But how often our anticipations of boiled and roasted feasts have been blighted. We have found that the chestnuts were like the manna which fed the children of Israel in the wilderness, "When we left of them until the morning they bred worms and became foul." There are numerous cases in this country where chestnuts in shipment have been seized and condemned under the provisions of the Food and Drugs Act. Usually the phraseology of the libel has been "because the shipment consisted in part of filthy animal substances, to wit, worms, worm excreta, worm-eaten chestnuts and decayed chestnuts." Altogether the loss to chestnuts from weevil injury is beyond computation.
The beetles which are the parents of the familiar worms in chestnuts are not commonly seen, or, if observed, they are not associated with the disgusting inhabitants of the nut kernels. These beetles represent in their structure a very interesting adaptation to a special end. The mouth is located at the tip of an enormously long snout, or proboscis, and the drill-like instrument is used for puncturing the thick covering of various kinds of nuts so as to admit the egg into the kernel upon which the young will feed. In some cases the mouth is situated at a greater distance from the eyes and other head appendages than is the anal extremity of the insect. There are in the northern part of this country two species which attack chestnuts, one which attacks hickory-nuts, one which attacks hazel-nuts and about a dozen which attack acorns. And here may be mentioned an interesting peculiarity of the feeding habit which is decidedly to the advantage of the nut-grower. Each species adheres closely to its own food plant. The hickory-nut weevil does not attack hazel-nuts nor the hazel-nut weevil hickory-nuts. None of the acorn-infesting species will seek for food in the nuts of chestnut, hickory or hazel. Once the chestnut weevils are absent in a locality, there is no chance that oak trees will serve as a means of spreading the weevils back into the locality. So closely confined are these weevils to their particular food plants that many of them distinguish between the different species of oak and will oviposit only in certain kinds of acorns.