Abstract

Outbreaks of folivorous insects often result in complete host plant defoliation. Insects on defoliated plants may suffer higher mortality, slower growth or reduced fecundity as a result of induced host plant resistance and depletion of preferred resources. This thesis focuses on effects of defoliation-mediated variability in host plant quality on the pinefeeding insect Bupalus piniarius (Lepidoptera, Geometridae).
B. piniarius prefers to feed on mature Scots pine (Pinus sylvestris) needles throughout the larval period. Depletion of the preferred resource caused by defoliation negatively affected larval performance, i.e. larvae on current-year needles suffered higher mortality and resulted in lighter pupae than conspecifics on branches containing both current-year and mature needles. There was no support for the induced resistance hypothesis. On the contrary, larvae feeding on previously defoliated branches performed better than those feeding on non-defoliated branches suggesting induced host susceptibility. Crowding associated with high population densities enhanced larval performance.
Ovipositing females showed a distinct preference for mature needles. When confined to current-year needles, females responded with reduced realised fecundity and frequently placed their eggs in an unsafe manner, i.e. on needle scales and stacked in several layers on top of each other. Due to their misplacement some eggs fell off the plant. Only a small fraction of larvae hatching from eggs on the ground was able to recolonise the host tree.
In order to estimate possible errors in forecasts of B. piniarius population changes due to random weather events, effects of low temperature and delayed mating on realised fecundity were studied in the laboratory. At 10 °C B. piniarius oviposition was almost completely arrested. Females that experienced 10 °C for four days had lower fecundity and increased longevity compared with females raised at constant 20 °C. Four days delayed mating at 20 °C had no effect on fecundity but affected fertility, i.e. 30% of eggs were laid before mating.
​​​​​​​It is concluded that defoliation-mediated variation in host plant quality can significantly influence B. piniarius population dynamics under outbreak conditions.

Keywords

defoliation; egg loss; growth rate; insect outbreaks; insect - plant relationship; mutual interference; population behaviour; realised fecundity; recolonisation

Published in

Acta Universitatis Agriculturae Sueciae. Silvestria
2001, number: 176
ISBN: 91-576-6060-3
Publisher: Department of Entomology, Swedish University of Agricultural Sciences

SLU Authors

• Smits, Agnis

  • Department of Entomology, Swedish University of Agricultural Sciences
    

UKÄ Subject classification

Forest Science


Permanent link to this page (URI)

https://res.slu.se/id/publ/107974