Density Dependence of Egg Recruitment and Moth Dispersal in Spruce Budworms

Abstract

Egg recruitment quantifies the relative importance of realized fecundity and migration rates in the population dynamics of highly mobile insects. We develop here a formal context upon which to base the measurement and interpretation of egg recruitment in population dynamics of eastern and western spruce budworms, two geographically separated species that share a very similar ecology. Under most circumstances, per capita egg recruitment rates in these budworms are higher in low-density populations and lower in high-density populations, relative to the regional mean: Low-density populations are nearly always migration sinks for gravid moths, and dense populations nearly always sources. The slope of this relationship, measured on a log scale, is negatively correlated with migration rate, and ranges between 0 and −1. The steeper the slope, the more marked net migration. Using our western spruce budworm observations, we found strong evidence of density-dependent emigration in budworms, so migration is not simply a random perturbation in the lagged, density-dependent stochastic process leading to budworm outbreaks. It is itself statistically and biologically density-dependent. Therefore, moth migration is a synchronizing factor and a spread mechanism that is essential to understanding the development and expansion of spruce budworm outbreaks at regional scales in the boreal forests of North America.

Dynamics and Management of Rising Outbreak Spruce Budworm Populations

“…We observed, in each year of this study, but most strikingly in 2013, net immigration into lower-density populations, and net emigration away from higher-density populations (Figure 7). This pattern of inverse density-dependence of apparent fecundity is ubiquitous in spruce budworms [20], and its regional average and slope vary from year to year in response to factors that affect the migratory behavior of moths as well as to the variability of population density in the source region. We noted that apparent fecundity dropped sharply in populations where defoliation exceeds 20% (Figure 8a), a level at which the density of emerging adults starts to level off (Figure 5b), an indication of intraspecific competition for food.…”

“…This would lead to highly patterned spatial distributions of epicenters [4]. Once these initial outbreak epicenters appear in the landscape, they can become emitters of migrating moths that spread the outbreak by increasing the apparent fecundity of the surrounding low-density populations [20].…”

“…While this is biologically unlikely, it does indicate that 2013 was a particularly intensive migration year (high immigration, low realized fecundity of resident moths). As a result, the slope of the relationship between apparent fecundity and resident moth density in 2013 was very near the extreme of −1 (Figure 7d), which is a telltale sign of regionally random redistribution of eggs through extensive migration of ovipositing female moths (defined as panmixis and explained in detail in [20]). In 2014, the immigration rate was highest among the LSL sites (I 2013 = 0.25 ± 0.07 eggs per shoot), and realized fecundity was high (F 2013 = 33.8 ± 9.4 eggs per moth) (Figure 7e).…”

“…In 2012, when the developing outbreak in the LSL was just beginning and populations were generally low in the region, the immigration rate and the realized fecundity of resident moths (parameters I and F of Equation (5)) were low (Figure 7a). As a result, the relationship between apparent fecundity and resident adult density had a slope near 0, an indication of limited migration activity (Figure 7b) (see [20] for a thorough discussion of this topic). The same was true in 2014 among SOPFIM sites (Figure 7e,f).…”

“…A theoretical framework for the analysis and interpretation of egg recruitment in spruce budworm population dynamics was recently developed [20]. The analysis of egg recruitment is done in accordance to that work.…”