The pine looper (Bupalus piniarius L.) is one of the most harmful defoliating insects on Scots pine. During outbreaks trees are often completely defoliated. In general, leaves on defoliated trees often differ in biochemical composition compared with leaves on non-defoliated trees (Karban & Baldwin, 1997). Thus, future generations of insects feeding on defoliated trees may suffer higher mortality, slower growth or reduced fecundity than conspecifics feeding on non-defoliated trees (Haukioja, 1990). The pine looper shows strong preference for mature needles as food and egg-laying substrate. We, therefore, hypothesized that current-year needles are suboptimal food for pine looper. Pine looper larvae feeding on previously defoliated trees may show lower performance either because of induced resistance or because larvae have to feed on suboptimal food (current instead of mature needles). In addition, female realised fecundity may be reduced in the absence of the preferred substrate for oviposition, i.e. mature needles.