Long-term population dynamics of western tent caterpillars: History, trends and causes of cycles

Abstract

This is a story of unique long-term studies of the population ecology of a gregarious, cyclic forest insect, the western tent caterpillar (WTC), Malacosoma californicum pluviale. Early work by W.G. Wellington proposed that variation in the 'quality' and activity of larvae and moths influenced their population ecology. Our subsequent studies monitored six WTC populations over 29-50 years in south-western BC to determine the consistent characteristics of cyclic population dynamics. The six studied populations fluctuated more or less in synchrony with an eight to 11-year periodicity. Fecundity and tent size (an indication of early larval survival) increased with population increase and declined several years before the population peak. Fecundity and tent size were positively related to the population growth rate and declined before the population peak. Mortality from a baculovirus was high at peak densities, and the rate of population growth was negatively related to infection levels. Resistance to the virus varied among families and was higher following the epizootic at peak host density. Factors that might influence changes in fecundity were explored. Viral resistance was not related to moth fecundity, but sublethal effects as a result of surviving virus exposure could reduce fecundity. Declines in fecundity and tent size prior to the peak density could be a result of reduced foliage availability and quality from induced effects of larval feeding. Introduction and cropping experiments were unsuccessful at creating out-of-phase populations, and introduced insects appeared to carry the 'quality' of the source populations and declined synchronously with them. Warming temperatures influence the phenology of egg hatch and leaf development, but field experiments show that WTC larvae are resilient to this variation. No signal of an influence of a warming climate was apparent in long-term data. Longterm field observations indicate that changes in fecundity and viral infection can drive population cycles and inform the theory of cyclic dynamics. The early focus on variation among individuals was a prelude to the eco-evo thinking that has become accepted today but should include both genetic and phenotypic change as being relevant.

Keywords: Malacosoma californicum pluviale; baculovirus; eco‐evo; forest Lepidoptera; host resistance; long‐term data; phenotypic plasticity; population cycles.

FIGURE 1

Western tent caterpillars (WTC) bask in the sun on elongate tents (a) and compact tents (b). When temperatures are too warm, they move into the shade under the tent (c). Defoliation occurs at peak densities (d).

FIGURE 2

Six study sites in the lower mainland of British Columbia are indicated by red dots.

FIGURE 3

Population cycles in western tent caterpillar (WTC) in south‐western British Columbia as shown by the numbers of tents counted annually on three islands in the Georgia Strait of British Columbia (a) and two mainland populations (b). Mandarte (dark blue), Galiano (green), Saturna (purple) Islands and mainland populations, Westham (blue) and Cypress (red). Black lines indicate peak years for the Galiano population. Updated from 2015 for Galiano and 2013 for the other sites (Franklin et al., ; Myers, ; Myers & Cory, 2015).

FIGURE 4

(a–c) Three characteristics of western tent caterpillar (WTC) that change over the population cycles on Galiano Island (grey bars) are: (a) fecundity (blue) and tent size (early larval survival) (green), (b) infection by McplNPV (pink). Parasitism (c) (purple) varies erratically. (d–f) Linear regressions of: (d) population size (number of tents) and viral infection levels (r ² = 0.42, p < 0.0008), (e) levels of viral infection and the rate of population increase (loge N _t+1/N _t where N is the number of tents counted) (r ² = 0.57, p = 0.001) and (f) fecundity and the rate of population increase (r ² = 0.41, p < 0.0004). Fecundity and infection data updated from Myers and Cory (2015); parasitism from Myers and Cory (2013), tent size (unpublished data, Myers and Cory).

FIGURE 5

Immigration of western tent caterpillar (WTC) is indicated by the intermittent populations at a coastal area of Galiano Island (red dots) with the population data from Mandarte indicative of potential source populations in grey bars. Data were not collected from Mandarte in 2020.