Population dynamics of the major North American needle-eating budworms.

Abstract

Data from six western States provided 1,251 life tables representing western spruce budworm, Chodstoneura occidentalis Freeman. These data provided projection capabilities for defoliation and successive budworm densities, as well as a basis for comparing survival rates among the three principal North American needle-eating budworms (western and eastern spruce budworms, and the jack pine budworm). Several modifications are suggested in current methods for managing budworm-susceptible forests, and suggestions are provided for further studies on the budworm life systems.Keywords: Western spruce budworm, eastern spruce budworm, jack pine budworm, life table, population dynamics, survival, site and stand attributes, interstand influences, year-to-year changes, insecticide treatment, weather, Choristoneura, predators, birds, ants, density dependence, host defenses, hazard rating. Extended, AbstractThe data for this study were collected between 1959 and 1988 from nine projects in six western States (Washington, Oregon, Idaho, Montana, Colorado, and New Mexico). This data base provided both 1,251 life tables and hundreds of additional survival-rate fragments representing populations of the western spruce budworm, Choristoneura occidentalis Freeman. These data were analyzed to produce projection capabilities for defoliation and successive budworm densities (eggs, nominal fourth instars, emerging moths, and egg masses) as functions of preceding density, site and stand attributes, interstand influences, systematic year-to-year changes in survival, recent insecticide treatment, and weather. Some of these results were then used to compare survival rates in western spruce budworm with equivalent rates found in populations of the other principal North American needle-eating budworms--the eastern spruce budworm, C. fumiferana (Clem.), and the jack pine budworm, C. pinus Freeman.Across an outbreak, generation survival shows similar declines in the three budworms. For example, in each of the three budworm life systems, predation processes appear to be paramount in maintaining sparse densities. More specifically, in innocuous western spruce budworm populations, the combined effects of predaceous birds and ants provide a low stable equilibrium that can maintain populations at low densities indefinitely, at least in the Pacific Northwest. Apparently, the combined effects of these predators on large larvae and pupae is highest when budworm density reaches about one or two fourth instars per m 2 of foliage. From this density to about 50 fourth instars per m 2, average survival from fourth instar to adults increases with increasing density, then declines again (from processes that are not usually related to predation) as density continues to increase. Trends in all three species also are heavily influenced by density-related mass flights by gravid moths.Other comparisons, however, show that the three life systems differ in some fundamental ways. For example, across a multiyear outbreak, drastic density-dependent reductions oc...

Herbivore-Induced Monoterpene Emissions From Coniferous Forests: Potential Impact on Local Tropospheric Chemistry

“…Several assumptions were made in calculating these rates: (1) only the current-year foliage was damaged (many herbivores prefer the youngest needle tissue of conifers (Miller et al 1991, Campbell 1993, Mc-Cullough et al 1993, Carisey and Bauce 1997, Clancy et al 1998a; (2) the herbivores damaged the needles according to the pattern we observed in the laboratory where only a distal portion of the needle is initially removed by the herbivore, leaving the remaining portion of the needle damaged on the tree for a period of time (Litvak and Monson 1998); (3) the change in monoterpene fluxes from wounded needles over an 8d period follows the pattern described in Litvak and Monson (1998); and (4) the simulations will only tell us the potential impact herbivory could have on local tropospheric chemistry during an outbreak.…”

“…In temperate forests during non-outbreak periods, defoliators regularly consume 5-15% of the leaf area produced (Schowalter et al 1986, Landsberg andOhmart 1989). Yet foliage losses can reach 25-100% in herbivore outbreak years (Kulman 1971, Furniss and Carolin 1977, Schowalter et al 1986, Mattson et al 1991, Campbell 1993.…”

Disturbance–climate relationships between wildfire and western spruce budworm in interior British Columbia

“…Second, warm and dry conditions characteristic of drought create a physical environment that increases larval survival (Rhoades , Mattson and Haack ), where larval survival, growth, and reproductive rates are greater during periods of low precipitation, foliar moisture content, and/or soil moisture (Cates et al. , Mattson and Haack , Clancy , Campbell ).…”

“…The program applies a set of userdefined criteria to identify sustained periods of reduced radial growth associate with insect outbreak intervals (Holmes and Swetnam 1996). Following the approach previously taken in this region (Axelson et al 2015) and in comparable environments (Swetnam and Lynch 1989, 1993, Ryerson et al 2003, Alfaro et al 2014, WSB outbreaks were identified where (1) a minimum threshold of eight years of below-average growth was detected and (2) there was a trend of reduced growth below À1.28 standard deviation. Standwide outbreaks were defined as occurring when at least 40% of host trees recorded defoliation (Flower et al 2014a).…”

“…Haack 1987), where larval survival, growth, and reproductive rates are greater during periods of low precipitation, foliar moisture content, and/or soil moisture (Cates et al 1983, Mattson and Haack 1987, Clancy 1991, Campbell 1993.…”