Increased nutrient availability decreases insect resistance in cranberry.
Herbivorous insects are considered to be a major problem in cranberry (Vaccinium macrocarpon Ait.) production. Spraying chemical pesticides is common practice, although beneficial insects could also be affected. We aimed to strengthen the resistance mechanisms that cranberry plants themselves use to combat insect pests by manipulating available nutrient levels. We hypothesized that cranberry genotypes respond differently from nutrient availability with regard to growth and defence against multiple herbivores. In the present study, six cranberry varieties (i.e. genotypes) were subjected to four different fertilizer rates. In a greenhouse setting, we assessed soil and leaf nitrogen concentrations; weight gain of the native pests spotted fireworm (Choristoneura parallela Robinson), sparganothis fruitworm (Sparganothis sulfureana Clemens) and the invasive pest gypsy moth (Lymantria dispar L.); levels of leaf proanthocyanidins (PACs); and plant growth. The results obtained showed that cranberry varieties differ in their insect resistance qualities, although this depended on the insect species identity; genotypic variation was observed in resistance against the invasive pest L. dispar but not against the two native insect pests. Overall, increased nutrient availability decreased plant resistance to the three herbivores (i.e. increased larval weight gain and reduced PAC levels), regardless of the cranberry genotype. Nutrient availability correlated negatively with PAC levels and positively with herbivore weight gain and plant growth, indicating a potential trade-off between growth and defence. Fertilization management to strengthen insect resistance in cranberry may contribute to the development of more sustainable pest management practices, especially for organic production.