Effect of temperature and leaf age on growth versus moulting time of a generalist caterpillar fed plantain (Plantago lanceolata)

Abstract

Abstract. The simultaneous effects of daytime temperature (20°C versus 30°C) and leaf age (new versus intermediate‐aged) on a generalist insect herbivore were examined. Fourth‐instar Spilosoma congrua caterpillars were tested on plantain (Plantago lanceolata), one of this lepidopteran species’host plants, for which the major defensive chemicals, iridoid glycosides (aucubin and catalpol), could be quantified. Cool temperature depressed amount of food eaten, amount of frass, and consumption and growth rates, and increased the proportion of time spent in the non‐feeding period (from head‐capsule slippage to ecdysis). Average iridoid glycoside concentration was 2.8% dry weight (d.w.) in new leaves and 1.6% d.w. in intermediate‐aged leaves. When fed new leaves, the caterpillars had a higher efficiency of conversion of ingested food to biomass and a higher growth rate than those fed intermediate‐aged leaves. Furthermore, the proportion of time spent in the non‐feeding period was prolonged by a diet of intermediate‐aged leaves. A second experiment showed that the percentage dry weight of aucubin, catalpol and total iridoid glycosides increased over 24 h in incubated, excised leaves, which meant that the caterpillars in the first experiment experienced somewhat higher iridoid glycoside concentrations than the levels in fresh leaves. Overall, these results indicate that this generalist should prefer new plantain leaves over older leaves even though new leaves contain higher concentrations of iridoid glycosides.