Effects of previous defoliation on pine looper larval performance

Summary 1 During outbreaks of the pine looper, Bupalus piniarius , its host, Pinus sylvestris , is severely defoliated. The larvae of this geometrid normally feed on mature needles. However, because trees are totally defoliated during outbreaks, the next generation is forced to feed on current‐year needles. 2 Bupalus piniarius larvae feeding on previously defoliated trees may show lower performance either because of induced resistance or because larvae have to feed on needles not normally fed upon (current instead of mature). 3 These hypotheses were tested in an experiment where larvae were reared on (i) shoots naturally defoliated the previous year, and thus, bearing only current‐year needles, (ii) non‐defoliated shoots where larvae had access only to current‐year needles, and (iii) control shoots with access to both current and mature needles. 4 There was no support for the induction hypothesis. Survival was lower on naturally defoliated shoots than on control shoots (81.3 vs. 90.9%), but survival was lower also on non‐defoliated shoots where larvae had access only to current‐year needles (78.8%). Data on larval feeding distribution showed a strong preference for mature needles. 5 Needle nitrogen concentration of current‐year needles was 38% higher on defoliated trees than on non‐defoliated trees. 6 It is concluded that defoliation affected larval performance primarily through the removal of the preferred type of needles and not because of an induced resistance. Effects of increased concentrations of allelochemicals in defoliated shoots, if present, were probably cancelled out by increased nitrogen concentrations.