The Forest Tent Caterpillar in Aspen Stands: Distribution and Density Estimation of Four Life Stages in Four Vegetation Strata
The forest tent caterpillar (Malacosoma disstria Hbn.) exemplifies the mobile defoliator that expands in vertical distribution during development from tree canopy to ground. For such insects the problem of estimating absolute numbers of each life stage is complex and has seldom been addressed. We investigated distribution of eggs, small larvae, large larvae, and cocoons (pupae) in overstory-tree, high-shrub, low-shrub, and ground strata in stands of quaking aspen (Populus tremuloides Michx.) in which defoliation ranged from virtually none to complete. The study took place in 11 stands in northern Minnesota during 5 yr, and in 5 stands in Upper Michigan during 1 yr. Trees were divided for insect counting into vertical crown-thirds, horizontal crown-halves, branches, branch parts, and bole. Insects were also counted on entire high shrubs, and in several sizes of low-shrub and ground plots. Eggs and small larvae occurred in the tree stratum regardless of population density. Vertical and horizontal distribution in overstory trees shifted through successive life stages and with different population densities. Density and defoliation intensity affected proportions of large larval and cocoon populations occurring in the three lower strata. Cocoons were maximally dispersed, with up to 97% occurring in high-shrub, low-shrub, and ground strata. Egg-mass numbers were satisfactorily estimated in trees from a sample of branches and tree dbh. Abundance of all life stages on trees was related to a power of tree dbh which at moderate to high population densities averaged 2.72 for egg masses, 4.23 for small larvae, 2.99 for large larvae, and, when defoliation did not exceed 60%, 2.33 for cocoons. These relations form the core of models devised for indirectly estimating absolute density of insects in the tree stratum from dbh and stand tables. At moderate to high densities, absolute density estimation in the four combined strata produced overall standard errors whose midrange percentages of estimates were 31 for eggs, 56 for small larvae, 40 for large larvae, and 38 for cocoons. Forest tent caterpillar distribution in aspen stands appears determined to a high degree by stand foliar area. For. Sci. 41(1):99-121.
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