Sub-lethal effects of monoterpenes on reproduction by mountain pine beetles

1 Plant defences may negatively affect the oviposition behaviour of insect herbivores. We exposed adult female mountain pine beetles Dendroctonus ponderosae (Hopkins) to monoterpene vapours and assessed their subsequent oviposition in lodgepole pine Pinus contorta. 2 Exposure to 31.25 and 125 p.p.m. of alpha-pinene and limonene for 24 h did not affect the survival or establishment of oviposition galleries compared with controls. Females had fewer, smaller eggs and therefore lower cumulative reproduction after exposure to 125 p.p.m. compared with the other treatments. Limonene negatively affected oviposition more than did alpha-pinene. 3 Female body condition, a measure of energetic state, influenced the responses (i.e. significant interactions between condition and monoterpene concentration). With exposure to 31.25 p.p.m., females in better condition tended to have fewer, larger eggs, whereas females in poorer condition tended to have more, smaller eggs, compared with controls. Overall, reproductive investment declined with body condition when exposed to monoterpenes. Egg size and number increased with female body size. 4 The findings of the present study may indicate the toxic effects of monoterpenes or may be a result of females choosing not to invest in adverse environments depending on body condition. In either case, reduced reproduction when exposed to high plant defences may be increasingly important as climate change affects plant defences.