Nitrogen and phosphorus fertilization of ivy geranium (Pelargonium peltatum) and impatiens (Impatiens wallerana) affects population levels of twospotted spider mite (Tetranychus urticae) and western flower thrips (Frankliniella occidentalis)

Plant nutritional status may affect pest incidence and severity. Ivy geranium (Pelargonium peltatum (L.) L'Her ex Ait.) ‘Amethyst 96’ and ‘Sybil Holmes’ were fertilized with 8, 16, or 24 mM nitrogen (N) and 0.32, 0.64, or 1.28 mM phosphorus (P) rates at 20% or 40% water loss levels. All treatments produced plants that met commercial quality standards, but the best quality was obtained with 8 to 24 mM N combined with 0.64 or 1.28 mM P at 20% water loss. Different plant tissue N and P resulting from these treatments served as the basis for plant-pest relationship studies.; Pest study 1 examined twospotted spider mite (Tetranychus urticae Koch) in ivy geranium fertilized with 8 or 24 mM N and 0.32, 0.64, or 1.28 mM P. All nutrient treatments resulted in commercial quality plants. Nitrogen had no effect on mite population; P had a marginal effect with more mites on 0.64 mM P. Twenty-four mM N and 0.64 to 1.28 mM P resulted in higher plant quality under mite pressure. Pest study 2 first investigated western flower thrips (Frankliniella occidentalis Pergande) population level and damage in response to these same nutrient treatments. ‘Amethyst 96’ is more resistant to thrips than ‘Sybil Holmes’. Eight mM N and 0.64 to 1.28 mM P resulted in more thrips. However, plant quality is a combination of pest level and plant compensatory ability: 24 mM N and 0.64 to 1.28 mM P also resulted in higher plant quality. Next, thrips life history was investigated at three P levels. Thrips had lower fecundity, longer development time and lower survival on ‘Amethyst 96’. Phosphorus at 0.64 mM resulted in shorter development time.; Pest study 3 examined thrips in impatiens (Impatiens wallerana Hook.f.) ‘Dazzler Violet’ fertilized with 8 or 20 mM N and 0.32 or 1.28 mM P. Severe damage occurred when plants were inoculated during the vegetative growth stage, but plants were not affected when inoculated during the reproductive stage when 63% of thrips were found feeding inside the flowers' nectariferous spurs. Nitrogen had no effect on thrips; P at 1.28 mM resulted in more adults from vegetative-stage inoculation.