Budworm densities

Our objective Is to examine some aspects of current plant herbivore theory using Douglas-flr (Pseudotsuga menzlesll) and western spruce budworm (Chorlstoneura occldentalls). Both plant and herbivore are widespread In western North America. Natural hosts of the budworm Include Douglas-flr, species of Abies, and, on occasion, other conifers (9). Variation In budworm density occurs on both a geographic and local scale. We have frequently observed differential defoliation In trees having overlapping crowns at sites In Montana, Idaho, and New Mexico. 

Studies to determine the level of resistance or susceptibility due to foliage quality and tree physical parameters of Douglas-flr to the budworm were conducted at the Boulder, Montana and Barley Canyon, New Mexico sites. The study at the Montana site Involved trees exposed to moderate budworm densities, while the study at Barley Canyon was done using budworm that were placed on trees that had very low natural densities of budworm. 

Natural budworm densities were determined by sampling 6 sprays, each 40 cm long, In the same quarter of the tree used to collect tissue for chemical analysis and to collect defoliation data. Densities were expressed as the average number of budworm larvae per 100 buds per tree. A visual estimate of the amount of defoliation eilso was made In the same area of the crown where the densities and needle tissue were collected. Since budworm may disperse from heavily defoliated trees, (Greenback, 1963) budworm densities from each tree were weighted by the level of defoliation that each tree sustained. This resulted In an Infestation Intensity measurement (dependent variable) which was subjected to multiple stepwise correlation analysis using various foliage quality and physical tree parameters as the Independent variables. Thirty-one parameters were used as Independent variables In this analysis. 

Resistance - Susceptibility Studies Using Natural Budworm Densities and Defoliation... 

Present methods for the detection and population survey of spruce budworm rely on the sampling of egg masses on tree foliage. Pheromone traps could probably detect moths at lower densities than the present method and this information could be used to prevent or decrease the development of an epidemic infestation of spruce budworm. However, a great deal of work is still required to correlate trap capture with moth population densities and with damage to trees. 

The spruce budworm is considered a "high density" pest since it occurs at damaging densities of > 1CT adults per hectare. In epidemic situations it occurs in densities sometimes orders of magnitude higher. Under such conditions, it is questionable that pheromone alone could suppress populations. However, recent work has shown some disruptive effects on reproductive behavior even at these high densities and... 

Mating disruption experiments with spruce budworm in laboratory, small-scale field tests and "semi-operational" field trials have recently been reviewed (27). Work to date, on the disruption of spruce budworm mating behavior has concentrated on the use of the primary components /Zll-14 Ald s (95-97% E), and the results indicate that some mating disruption does occur. There appears to be a positive correlation between the applied pheromone concentration and the percent disruption, and based on field cage studies, percent disruption is inversely related to insect density (27) as would be predicted (32). 

Previous tests of the aerial application of synthetic sex attractant of the spruce budworm resulted in reductions of up to 90% of males captured in traps baited with virgin females and/or synthetic pheromone. However, there has been no convincing demonstration of reduction in population density in the subsequent generation (33). 

Pheromone releaser distributions and/or point-source release rates have pronounced effects on the disruption of trap captures of spruce budworm (39). Small field plot studies with hand-placed releasers in moderate- to-high density budworm populations indicate an increase in trap disruption as the point sources of the synthetic pheromone are increased in release rate and decreased in number per unit area (Fig. 1). Identical pheromone dosages per plot were present in each treatment. Optimizing the releaser spacing and point-source release rate is therefore important and implies the need for a formulation re-design. This effect of releaser distribution and point-source release rate on trap disruption and mating disruption has been demonstrated in several insect species (40). 

Pheromones for control of budworm or other potential high-density, migratory defoliators have a role to play, but these epidemic pests present special problems and a great deal more research is required before pheromones will take their place in the integrated pest management of these insects. 

A biochemical study indicated that the cuticle of the DDT-resistant strain of tobacco budworm contains more protein and lipid than that from the susceptible strain (Vinson and Law, 1971). Also, there was evidence of increased sclerotization of the cuticle of resistant insects. This suggests an increase in density and hardness of the cuticle that probably decrease its permeability to insecticide molecules. 

Ludwig et al. (1978) proposed and analyzed an elegant model of the interaction between budworms and the forest. They simplified the problem by exploiting a separation of time scales the budworm population evolves on a fast time scale (they can increase their density fivefold in a year, so they have a characteristic time scale of months), whereas the trees grow and die on a slow time scale (they can completely replace their foliage in about 7-10 years, and their life span in the absence of budworms is 100-150 years.) Thus, as far as the budworm dynamics are concerned, the forest variables may be treated as constants. At the end of the analysis, we will allow the forest variables to drift very slowly—this drift ultimately triggers an outbreak. 

During the 1970 s the USDA Nicotiana tabacum germplasm collection was evaluated in field plots at the Clemson University Pee Dee Research and Education Center, Rorence, SC for their resistance to the tobacco hornworm, Manduca sexta (L,) (1), and the tobacco budworm, Heliothis virescens (F.) (2). During this investigation a large variation in leaf trichome types and density was observed. Johnson et al. (3,4) classified the major trichome types from the various tobacco types as simple trichomes without exudates, glandular trichomes without exudates, glandular trichomes with exudates, and... 

Simmons, G. A., Leonard, D. E. and Chen, C. W. (1975) Influence of tree species density and composition of parasitism of the spruce budworm, Choristoneura fumiferana (Clem.) Env. Ent., 4, 832-6. 

The male responds to one attractant pheromone component and is inhibited by two others. Surveys with small attractant traps have been shown to be extremely effective in catching male spruce budworms in areas where conventional larval sampling has failed to locate any insects. Again, the potential use of the attractant is considerable, but implementation of the system for monitoring low density populations requires calibration of the catches with population density which must await the decline of the current extensive infestations to endemic levels. (570). Sanders (570) has summarized the results of a semi-operational attempt at disruption of spruce budworm mating by the synthetic attractant in Ontario in 1977 Semi-operational trials of the aerial dispersion of the synthetic attractant of the eastern spruce budworm from aircraft have... 

Our efforts on western spruce budworm have thus far been limited to survey applications. Specifically, we have been attempting to correlate moth captures (in pheromone-baited traps) with larval densities and/or defoliation on a series of test plots. We would like to expand this effort in the future, and also explore use of the budworm attractant for control purposes. 

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