Host plant nutritive value and variable number of instars in a sawfly, Diprion similis.
In measurements in Canada of the utilisation of dry matter and nitrogen by fifth-instar (penultimate) female larvae of Diprion similis (Htg.), leaves of jack pine (Pinus banksiana) and Scots pine (P. sylvestris) were found to be nutritionally superior to those of red pine (P. resinosa) or white pine (P. strobus). When the larvae were fed on leaves of low nutritive value and low nitrogen digestibility, some larvae entered a feeding sixth instars (supernumerary larvae), but only five feeding instars occurred on leaves of high nutritive value and high nitrogen digestibility. Topical application of a juvenile hormone preparation to 3-day-old fifth instar larvae prolonged the larval stage without introducing an additional instar.