Biogenic gradients of CO2 and H2O and oviposition by the small white butterfly (Pieris rapae L.) in cages

Despite considerable research into the impacts of herbivores on plant gas exchange activity, there is minimal knowledge of photosynthesis and transpiration of plants at the time arthropod oviposition occurs. During oviposition, the small white butterfly (Pieris rapae L.) use visual and chemosensory cues to allocate most eggs to larger, more nutritional host plants (Brassica oleracea L.) with characteristically higher gas exchange activity. However, it is not known whether biogenic gradients of CO2 or H2O have a direct influence on host plant choice. During caged assays, watered cabbage plants with higher transpiration/photosynthetic rates were preferred, but only during extreme water deficits that led to wilting. Shorter assays permitted gas exchange to be manipulated but did not reveal preferences for plants exposed to elevated levels of water or PAR and, therefore, with higher gas exchange activity. These findings support previous field observations that there is no mechanistic basis to female preferences for plants with higher gas exchange activity, although associated characteristics relating to the plant status (in this case superior water relations) were preferred. The implications of using caged assays (some of short duration) to decipher egg-laying preferences are considered.