TREHALULOSE SYNTHESIS BY WHITEFLIES: A MECHANISM FORSURVIVAL ON WATER-STRESSED HOSTS
Rufus Isaacs1, David N. Byrne1 and Donald L. Hendrix2
1Department of Entomology, University of Arizona, Tucson,Arizona 85721,
2USDA-ARS, Western Cotton Research Laboratory, 4135 E. Broadway,Phoenix, Arizona 85040, USA.
The unusual disaccharide, trehalulose, is the major excretory sugarcomponent in the whitefly, Bemisia tabaci, although it is absentin its diet. The physiological function of this synthesis has been unclear,although a role in osmoregulation has been proposed. We tested the hypothesisthat plant water stress would lead to differential trehalulose excretionby B. tabaci. Melon plants, Cucumis melo, grown under highand low watering regimes produced plants with strong developmental responsesto the treatments. There was, however, no effect on the whitefly developmentor on the weights of male or female adults. Honeydew production measurementsshowed that feeding rates were significantly lower on stressed plants.Chemical constituents of phloem sap and honeydew samples were then analysedto determine why the feeding behaviors differed. The honeydew of insectson water-stressed plants contained greater concentration of carbohydratethan those on unstressed plants, and was composed of a significantly greaterproportion of trehalulose. This isomerization of a less labile sugar fromthose in the diet suggests that B. tabaci uses a mechanism of osmoregulationto actively maintain its internal water status on plants with high sugarloading in the phloem sap. We conclude that physiological and behaviouralmechanisms enable exploitation of water-stressed host plants that providea phloem sap diet with relatively high levels of amino acids.