These results indicate clearly the effectiveness of DDT in chestnut weevil control, in spite of numerous discrepancies brought about by the small number and variability of the trees available for the tests. As might be expected, programs of three applications were more effective than those of only one or two. Of the single applications, those put on during the latter half of August were much more effective than those made during the first half of September, presumably because most of the eggs had been laid by the early part of September.
These experiments gave fairly exact information on the relative abundance of the two species of weevils. At Glenn Dale the small chestnut weevil constituted 92 to 98 per cent of the population; at Elkton, 61 per cent.
The matured nuts that fell from count trees were collected daily, and one-third of each lot collected was used as a sample for determining the percentage of wormy nuts. It was possible, therefore, to obtain a rough estimate of the numbers of larvae produced on each tree. In 1946, from 1,863 nuts on a tree sprayed three times, 429 larvae emerged; and from a comparable unsprayed tree having 2,610 nuts 16,494 larvae emerged. In 1947, 1,350 larvae were produced on 9 trees with an average crop of 1,361 nuts sprayed three times, compared with 14,505 larvae from 2,922 nuts on an unsprayed tree. These figures indicate that DDT sprays bring about large decreases in the numbers of weevils and that the proper use of DDT sprays on all host trees over a period of a few years would doubtless reduce the infestation to a point where fewer applications would be necessary for effective control of the chestnut weevils.
+Tentative Recommendations+
For the benefit of those who wish to try DDT for chestnut weevil control, the following tentative recommendation is made:
Thoroughly apply +to all parts of the tree+ 2 pounds of DDT in 100 gallons of water. For example, use 4 pounds of a wettable powder that contains 50 per cent of DDT, or 8 pounds of one that contains 25 per cent of DDT. Make three applications, the first about 30 days before the first nut is due to drop, and the second and third after intervals of 12 days. Unless the entire bur, especially that portion near the stem end where most of the feeding punctures are made, +is thoroughly covered+ with a film of DDT, the weevils may feed without being affected by the insecticide. In handling DDT, one should use the same care as with such well-known poisons as lead arsenate, Paris green, calcium arsenate, and nicotine.
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Mr. Gravatt: I might say that Mr. Van Leeuwen has used only a small section of our experimental orchard, and right near-by would be large sections not used. The weevils are not killed quickly by the DDT, they are somewhat resistant, and so we think quite a number of weevils come over and deposit eggs before they are killed by this DDT, because they don't lose any time getting to work on the nuts. He hopes to have much better results where the entire orchard is sprayed. This year we sprayed our entire orchard twice, and it is a real pleasure to go out there now and gather up nuts and not be eating weevils when we do eat them.
President Davidson: Well, Mr. Gravatt will now give us a talk on Diseases Affecting the Success of Tree Crop Plantings, and I am sure we all are on our toes to hear about that. Mr. Gravatt.
Mr. Gravatt: I only ask a few minutes to show a few slides.