Journal of Insect Behavior, Vol. 14, No. 4, 2001 Does the Small White Butterfly (Pieris rapae L.) Aggregate Eggs on Plants with Greater Gas Exchange Activity? A. Mark Langan, 1,2 C. Philip Wheater, 1 and Peter J. Dunleavy 1 Accepted January 10, 2001; revised February 22, 2001 Few studies have investigated insect egg-laying preferences in relation to pho- tosynthesis or transpiration of their host plants. It has been suggested that in- travarietal preferences of the small white butterfly (Pieris rapae L.: Pieridae) include larger plants with characteristically higher transpiration rates. Inter- estingly this species, like many other Lepidoptera, may detect biogenic CO 2 gradients associated with photosynthesis. We studied egg-laying preferences in working farm environments examining relationships among host choice, plant gas exchange activity, and plant size. Females discriminated between plants in monocultures on the basis of height. A balance of pre- and post–alighting preferences resulted in plants of medium size receiving eggs. Post–alighting preferences led to plants, but not alighted leaves, with higher rates of photo- synthesis supporting eggs. These findings do not support a mechanistic basis for the use of gas exchange activity during host selection but, for the first time, indicate the greater physiological activity of crop plants that ultimately received the eggs of a pest insect. KEY WORDS: host plant selection; Pieris rapae; photosynthesis; transpiration; brassica crops. 1 Department of Environmental and Geographical Sciences, Manchester Metropolitan Univer- sity, Manchester M1 5GD, UK. 2 To whom correspondence should be addressed. Fax: 0161 2476318. E-mail: m.langan@mmu. ac.uk. 459 0892-7553/01/0700-0459$19.50/0 C  2001 Plenum Publishing Corporation