Chestnut Weevils and Their Control with DDT
E. R. VAN LEEUWEN
United States Department of Agriculture, Agricultural Research
Administration, Bureau of Entomology and Plant Quarantine, Division of
Fruit Insect Investigations.
Failure of the American chestnut to resist the chestnut blight has resulted in the planting of a few blight-resistant species obtained from foreign lands. These foreign chestnuts would now be planted more extensively in certain districts, were it not for the fact that the nuts are injured by two species of weevils, for which heretofore there has been no practical control.
The 1947 season marks the fourth year of the experimental use of DDT for control of the chestnut weevils. During these years our knowledge of the spray and how best to use it has been advanced by conducting laboratory and field tests. Unfortunately, few chestnut orchards now exist in the Eastern States, and the scattered plantings consist mostly of a large number of Asiatic seedlings, some of which had to be top-worked to other Asiatic species and varieties. Many of these trees are grown for ornamental, shade, or timber purposes rather than for nut production. Owing to these conditions and to a series of spring frosts since 1945, it has been impossible to conduct insecticide experiments on an adequate basis of replicated plats.
Although much is to be learned regarding time of application of the sprays and the proper dosage, the use of DDT can be recommended as a standard practice, because it has proved highly valuable in protecting chestnut trees from heavy losses due to the chestnut weevil. It is the purpose of this paper to discuss some of the experiments that have been made with DDT and the observations made on the time of egg deposition.
+Nature and Extent of Injury+
The worms attacking chestnuts are the larvae of two very similar species of weevils, one larger than the other. The adults are medium-sized beetles having extremely long, slender beaks. With these they drill through the husk of the nuts, making openings through which they insert their eggs into the nuts. From these eggs the familiar worms develop. Weevil injury varies greatly in different chestnut-growing localities. It is not unusual for 50 to 75 percent of the nuts to be wormy, and often infestation reaches 90 to 100 per cent. The small weevil does the most damage, but there are indications that this may not always be true. Because the mouth parts of the adult are situated at the end of an extremely long and slender beak, it can obtain most of its food from beneath the surface of the host plant. For this reason, stomach poisons applied to trees have not been eaten by these weevils, and hence have been of no practical value. As DDT kills by contact, it is necessary only for the body of the insect to come in contact with DDT.
+Life Histories of the Weevils+
In the vicinity of Beltsville, Md., the adults of the large chestnut weevil[8] leave the soil about August 15. The date will vary, of course, with season and locality. Both males and females soon begin to feed by piercing the burs with their long beaks. Mating begins soon after the weevils collect on the trees, and egg laying follows shortly. The eggs hatch within a few days and the worms develop within the nut. A few of the worms will complete their growth and leave before the nuts fall, but most of them emerge from the nuts after they have fallen. The worms then enter the soil, where they build cells and remain until they change to pupae the following summer. This weevil has a one-year cycle, or one generation a year.