Variegated plants are widely propagated for ornamental purposes, being reported in 135 plant families. Managing arthropod pests that feed on these variegated cultivars is likely to be difficult because these slower growing plants are less capable of replacing plant tissue than faster-growing green plants. This problem is compounded by findings that indicate sucking arthropod pests can have greater capacities for population increase on slow-growing variegated cultivars. Both defensive and nutritional hypotheses have been used to explain this phenomenon. With portions of leaves unable to photosynthesize, variegated plants are likely to have less carbon assimilate to allocate to the production of defensive compounds than green plants. Alternatively, like plants grown in the shade, variegated plants compensate for reduced photosynthesis by allocating more of their biomass to the production of leaf area. As such the improved response of sucking insects may be due greater mobility of amino nitrogen and carbohydrates in the phloem to fuel the production of more leaves.
We examined the amino acid and sucrose content of stem phloem excised from red-variegated, yellow-variegated and green-leafed Coleus stem phloem and their relation to the life history characteristics of citrus mealybug to test these defensive and nutritional hypotheses. Our data soundly reject the nutritional hypothesis. Green leafed plants had more than twice the concentrations of amino acids in phloem than did the variegated cultivars. Analysis of amino acid composition indicates that green leafed plants had greater levels of shikimic acid precursors than did the variegated plants. Furthermore, life history characteristics of citrus mealybugs were adversely affected by increasing amounts of shikimic acid precursors. In conclusion we find that the improved suitability of variegated plants is more consistent with carbon-based theories of plant defense.
The ESA 2001 Annual Meeting - 2001: An Entomological Odyssey of ESA