A treatment with the insect growth regulator (IGR) CGA 34 301 (ethyl 5-(4'-phenoxy)phenyl-pent-2-enoate) on larvae of Psylla piri (L.) induces the formation of extra larval instars and adultoids a t metamorphosis. Such abnormal insect forms to not develop into reproductive adults. Predator bugs, seem not to be affected by IGR treatments in the field. Therefore, we see a reasonable chance for IGRs to became new useful tools in integrated control programms against pear psyllids.2. ang. Ent. 78 (1975), 313-316