Susceptibility of Eastern White Pine to White Pine Weevil Attack
White pine weevil resistance studies in New York with selected eastern white pine seed sources
Note by D.P. Connola, M.J. Birmingham and R.E. Davis
The study tests the hypothesis that there are genetic differences in the susceptibility of eastern white pine to white pine weevil attack associated with the geographic origin of the host. Tests were made with 4 known seed sources in small cages with potted trees and in field plots. None of the results supported the hypothesis but individual trees appeared to be resistant. The seed sources were from heavily weeviled and sparsely weeviled areas.
Discussions and Conclusions
Results of our tests indicate that the environment rather than geographic origin of eastern white pine seed appears to be the controlling factor governing the extent of weeviling in an area. Two factors support this conclusion: (1) The results of the small cage tests, although limited, indicated no significant differences in the susceptibility of the different seed sources to weevil attack. (2) All the seed sources in the field plots were attacked at about the same rate depending on where they were growing.
Studies by Connola and Beinkafner (1976) with the 4 seed sources planted in large outdoor cages adjacent to one another showed no significant differences in tree dimensions among the seed sources but there were significant differences between the dimensions of weeviled trees as compared to non-weeviled trees. The non-weeviled trees in each source had smaller dimensions. The studies reported here not only confirmed those results but also showed by computer analysis that bark thickness was the most important factor in determining whether or not a tree may be weeviled.
The analysis of tree dimensional data from the field plots showed significant differences in growth in the same seed sources planted in different locations. The differences not only reflected the effects of the environment on eastern white pine growth but also determined the susceptibility of the trees to weevil attack.
It may be mentioned here that studies by Connola and Wixson (1963) on 266 sample plots 0.04 hectares in size scattered throughout most of New York State showed that the heaviest weeviling occurred on plots with poor internal soil drainage, especially on plots with impermeable hardpan within the first 3 feet of soil depth. The area of heavy weeviling in the study reported here had poor internal drainage with hardpan within the first 3 feet of soil depth. Apparently such a situation creates a stress on the trees making them more susceptible to weevil attack. On the other hand, the area of sparse weeviling in the study reported here had good internal soil drainage except for one plot, yet that plot did not become more weeviled than the others. Thus, there are other factors in the environment which affect rate of weeviling.
Painter (1966), who had nearly 40 years of experience at Kansas State University in breeding plants resistant to insect attack, stated that resistance of economic value has been secured and used without knowing the reason for the resistance, which he states is usually found to be heritable. Thus, a plant that exhibits resistance when exposed to heavy attack should be fully utilized in the development of a resistant strain.
On the basis of our findings in this study, it is concluded that the best course to follow in developing a resistant strain of eastern white pine is to vegetatively propagate cuttings from trees which have not become weeviled in areas where weeviling is heavy. To do this successfully, the following guidelines are proposed: (1) Select trees from a stand in which over 90 per cent of the trees have been weeviled. (2) Select only trees that have made good growth and have good form. (3) Select trees which are over 5 meters tall. The latter recommendation is based on studies by Connola and Wixson (1963) which showed that in almost 1,000 trees over 15 meters tall, sampled throughout New York, weeviling began when they were about one and one half meters tall, reached a peak when they were about 5 meters tall and then declined. Thus, it is assumed that any susceptible tree which is exposed to weevil attack will have been weeviled by the time it is 5 meters tall.
The above recommendations are now in practice in New York in working toward the development of a resistant strain of eastern white pine. A clonal orchard has been established in an area where it will be subject to weeviling.