Seasonal Variation in Plant‐Insect Associations: Utilization of Solanum Dulcamara by Leptinotarsa Decemlineata

In laboratory studies, Solanum dulcamara (nightshade) is one of the better host species for survival and growth of the Colorado potato beetle, Leptinotarsa decemlineata. The dynamics of L. decemlineata on S. dulcamara in the field, however, are far different than would be predicted from laboratory results. Therefore, a number of experiments were performed to determine the suitability of S. dulcamara for L. decemlineata under seasonal, field conditions. Field experiments showed that between 40 and 67% of first—generation adult females entered diapause without reproducing when fed S. dulcumara as both larvae and adults, although no adults entered diapause when fed S. tuberosum in both life—stages. Dispersal by first—generation adults from S. dulcamara was not observed in mark—recapture experiment. Survival of L. decemlineata on S. dulcamara did not differ significantly from survival on S. tuberosum during the part of the season corresponding to the development period of first—generation larvae. Survival on S. dulcamara was two—thirds lower during part of the season corresponding to the development period of the second—generation larvae, however. Emergence mass and larval development time varied less over the season on S. dulcamara than did survival. Analyses of extractable leaf protein, free amino acids, and glycoalkaloids showed significant seasonal variation in the concentrations of these S. dulcamara phytochemicals which are known to affect L. decemlineata feeding growth and survival. Multiple regression analyses between phytochemical concentration and larval survival showed that survival was significantly] positively associated with leaf nutritional quality (extractable protein and one free amino acid) during the first generation, and significantly negatively associated with leaf glycoalkaloid content during the second generation. Thus the seasonal variation in the suitability of S. dulcamara for L. decemlineata survival was most strongly associated with the two seasonally most variable S. dulcamara phytochemical factors. The unique and independent contribution of variation in either one to the variation in L. decemlineata survival, however, could not accurately be predicted without considering the simultaneous, independent variation in the other.