Seasonality in neotropical populations of Plutella xylostella (Lepidoptera): resource availability and migration

Abstract

The present study aimed to verify (1) whether seasonal increases in neotropical populations of Plutella xylostella are directly provoked by regular influxes of migrants, and (2) whether temporal variation in food availability is the ecological process behind such predictable events. Over 3 years, plants that P. xylostella prefers were cultivated and irrigated in order to provide a continuous and abundant supply of food. Nevertheless, seasonal oscillations in the population of the herbivore still occurred. The hypothesis of seasonal availability of host plants could not explain the population pattern. In April, when the insect was practically extinct from the area, an artificial infestation (immigration) with 10,000 pupae established a precocious population. Therefore, the start of the natural cycles of population growth, during July–August, seems to be due to external factors, rather than an improvement in local conditions for resident individuals. In the beginning of the natural cycles, the increase in the density of adults significantly preceded the increase in immatures. Plutella xylostella does not diapause, and therefore immigration is the proximate cause of the seasonal population increases. Hypotheses about local factors are suggested to explain the decreasing phase of the predictable population cycles.

Photoperiodism and seasonality in neotropical population of Plutella xylostella L. (Lepidoptera: Yponomeutidae)

“…In Southwest Brazil, P. xylostella is more abundant in October (Campos et al 2006) within a narrow range of photoperiods (11-13h of light per day). On the other hand, P. xylostella has been seldom found in the same day-lengths during the fi rst half a year.…”

“…However, if individuals do not respond to changes in day-length, the direct causal relationship between photoperiod and population dynamics does not exist because changes in population size are due to physiological and behavioural responses of individuals (Kingsolver 1989). The infl uxes of immigrants in AugustSeptember (Campos et al 2006) and a supposedly migratory effl ux in November-December are not caused by photoperiod among neotropical populations of DBM.…”

“…The sampling procedure and descriptive statistical models for seasonal cycles among neotropical populations of DBM were reported elsewhere (Campos et al 2006). Brassicas are ephemeral plants cultivated during the whole year in the tropics.…”

“…Neotropical populations of DBM have seasonal cycles of growth and decrease. These cycles have higher densities between late August and early November and predictable infl uxes of migrants are the proximate cause of the population growth (Campos et al 2006). The environmental stimuli potentially triggering the migratory behaviour of DBM have received little attention (Shirai 1993, Begun et al 1997, Campos et al 2004, althougth seasonal change in the day-length was believed to stimulate lower fecundity in this species (Harcourt & Cass 1966, Babushkina 1990, Leather 1995.…”

“…The environmental stimuli potentially triggering the migratory behaviour of DBM have received little attention (Shirai 1993, Begun et al 1997, Campos et al 2004, althougth seasonal change in the day-length was believed to stimulate lower fecundity in this species (Harcourt & Cass 1966, Babushkina 1990, Leather 1995. Because migration and consequent population oscillation of P. xylostella are predictable (Campos et al 2006), the photoperiod may act as a stimulatory signal to the insect takes off. To check this hypothesis, I verifi ed whether the preimaginal and reproductive development of DBM is inhibited by artifi cial day-lengths that are equivalent to the periods of natural population growth and decrease.…”