Does the small white butterfly (Pieris rapae L.) aggregate eggs on plants with greater gas exchange activity?

Few studies have investigated insect egg-laying preferences in relation to photosynthesis or transpiration of their host plants. It has been suggested that intravarietal preferences of the small white butterfly (Pieris rapae L.: Pieridae) include larger plants with characteristically higher transpir ation rates. Interestingly this species, like many other Lepidoptera, may d etect biogenic CO2 gradients associated with photosynthesis. We studied egg -laying preferences in working farm environments examining relationships am ong host choice, plant gas exchange activity, and plant size. Females discr iminated between plants in monocultures on the basis of height. A balance o f pre- and post-alighting preferences resulted in plants of medium size rec eiving eggs. Post-alighting preferences led to plants, but not alighted lea ves, with higher rates of photosynthesis supporting eggs. These findings do not support a mechanistic basis for the use of gas exchange activity durin g host selection but, for the first time, indicate the greater physiologica l activity of crop plants that ultimately received the eggs of a pest insec t.