Influence of delayed density and ultraviolet radiation on caterpillar baculovirus infection and mortality

Abstract

Infectious disease is an important potential driver of population cycles, but this must occur through delayed density-dependent infection and resulting fitness effects. Delayed density-dependent infection by baculoviruses can be caused by environmental persistence of viral occlusion bodies, which can be influenced by environmental factors. In particular, ultraviolet radiation is potentially important in reducing the environmental persistence of viruses by inactivating viral occlusion bodies. Delayed density-dependent viral infection has rarely been observed empirically at the population level although theory predicts that it is necessary for these pathogens to drive population cycles. Similarly, field studies have not examined the potential effects of ultraviolet radiation on viral infection rates in natural animal populations. We tested if viral infection is delayed density-dependent with the potential to drive cyclic dynamics and if ultraviolet radiation influences viral infection. We censused 18 moth populations across nearly 9 degrees of latitude over two years and quantified the effects of direct and delayed density and ultraviolet radiation on granulovirus infection rate, infection severity, and survival to adulthood. Caterpillars were collected from each population in the field and reared in the laboratory. We found that infection rate, infection severity, and survival to adulthood exhibited delayed density-dependence. Ultraviolet radiation in the previous summer decreased infection severity, and increased survival probability of the virus. Structural equation modelling found that the effect of lagged density on moth survival was mediated through infection rate and infection severity, and was 2.5 fold stronger than the effect of ultraviolet radiation on survival through infection severity.

Altered precipitation dynamics lead to a shift in herbivore dynamical regime

“…However, laboratory rearing of A . virginalis from multiple monitored populations suggested a delayed‐density‐dependent rate of viral infection (Pepi et al 2021) as is the case in many Lepidoptera (Anderson & May, 1980; Myers & Cory, 2013, 2016).…”

“…Caterpillar populations at the Bodega Marine Reserve and other sites often exhibit high mortality rates after high population density years due to a granulovirus. Monitoring at Bodega and other sites has shown delayed density‐dependent infection and mortality rates due to granulovirus (Pepi et al 2021).…”

“…We included precipitation as a covariate based on previous knowledge that precipitation was important to dynamics. We included direct density‐dependence based on previous detection in time series analyses (Karban & de Valpine, 2010), and delayed density‐dependence based on wavelet periodogram results and the observation of delayed density‐dependent mortality from granulovirus in field studies (Pepi et al, 2021). Models with and without a threshold ( C ) were compared using AIC.…”

Hilltopping influences spatial dynamics in a patchy population of tiger moths

“…This local bi-directional movement produced a positive relationship between larval patches and nearby hilltop moth counts (Figure 2), and a similar positive relationship between hilltop moth counts and nearby larval counts in patches in the following year (Figure 3). Both estimates of larval and adult density are imperfect predictors of the next stage, since there is mortality (e.g., due to disease [30], predation [33]) during the intervening periods that is likely density-dependent. Such density-dependent processes have high potential to remove any correlation between these stages, and therefore the effects of movement were likely substantially stronger before density-dependent mortality.…”

“…We also included direct and delayed density dependence terms, intended to represent direct and delayed effects of baculovirus infection on dynamics that we have previously documented [26,30]. The model comparison started from the following full model structure:…”