Effects of light intensity on Solanum mauritianum (Solanaceae) morphological and chemical traits and the performance of its biological control agent Gargaphia decoris (Hemiptera: Tingidae)

The lace bug Gargaphia decoris Drake (Hemiptera: Tingidae) has been released in New Zealand for the biological control of woolly nightshade Solanum mauritianum Scopoli (Solanaceae). It has been observed that at release sites, G. decoris more commonly infests plants growing in shaded locations, compared to plants growing under full sunlight conditions. To explore the mechanisms for this preferential selection, we tested the effects of low shade (LS) and high shade (HS) on the physical and chemical traits of S. mauritianum and the performance of G. decoris under glasshouse conditions. New leaves of older plants under LS conditions were smaller, thicker, had longer trichomes, lower water content and lower specific leaf area compared to HS leaves. Leaves of LS plants increased their C:N ratio but total glycoalkaloid concentration decreased as the plants grew older. High shade plants were taller, with higher shoot-to-root ratios, and new leaves of older plants had higher photosynthetic efficiency compared to LS plants. Gargaphia decoris showed no discrimination between HS and LS leaves for oviposition or feeding. We observed no differences on first- and second-generation G. decoris performance traits, such as fecundity (e.g., eggs/female), fertility (% egg hatch) and adult survival. However, as the plants grew older, second generation G. decoris females failed to oviposit on LS plants and displayed smaller bodies compared to the females that developed feeding on HS plants. We concluded that light intensity did affect physical and chemical traits of S. mauritianum plants and these effects influenced G. decoris performance. Although, the key factor behind the differential performance of G. decoris under contrasting light conditions remains unclear, we believe that plant glycoalkaloid concentrations contribute to G. decoris host plant selection.