To determine if defoliation of a woody plant affects foraging by folivorous insects, we examined the infection rate (number of leaves damaged per total number of leaves sampled on marked stems) ofAnthyllis cytisoidesunder three experimental treatments: 10, 50 and 90% plant defoliation. Observations were made for three age classes, established by trunk base perimeter (equal to or lower than 11 cm, between 11 and 20 cm, larger than 20 cm). Plants respond positively to artificial defoliation by increasing total vegetative length of the stem and total inflorescence length. This response is most evident in young individuals. Response to herbivory was measured as overall infection rate and also as infection rate by different feeding guilds—chewing, mining, or sucking insects. We found that increased defoliation elicited increased resistance of leaves to insect attack. This was particularly evident in young plants. Different insect guilds respond in different ways. Attack by chewing insects declines with defoliation for all plant age classes; only sucking insects which feed on the oldest plants reduce feeding rate with plant defoliation. Finally, mining insects present the opposite trend in young and senescent plants.

Thanks to Jos´e Alonso for enabling us to work at his farm. Dr J. Emlen made valuable suggestions on the manuscript. We are also grateful to Prof. S.J. McNaughton, Dr S.J. Milton and Dr R.J. Marquis for critically reading the manuscript and helpful suggestions. Lorenzo Garc´ıa, Javier Ortiz and Monica Rueda helped us during the data collection. The work was supported by the Comisi´on Interministerial de Ciencia y Technolog´ıa, project number AMB93- 0777-C02-01, and MEDALUS II (Mediterranean Desertification and Land Use) collaborative research project. MEDALUS II was founded by the EC under its Environment Programme, contract number EV5V 0128. The support from both programmes is gratefully acknowledged.

Peer reviewed