The silverleaf whitefly (SLW), Bemisia argentifolii (Bellows and Perring), has been a severe pest of numerous field and vegetable crops for many years. Nevertheless, little has been reported on the complex interactions between SLW and its host plants. This study was aimed at investigating the effects of SLW on cotton (Gossypium hirsutum L.) photosynthesis and productivity under various irrigation and N fertilization regimes. Cotton plants (cv. Siv'on) were examined during an entire growing season in two factorial‐designed screenhouse experiments consisting of two irrigation regimes (I) × two N application rates × two SLW levels (noninfested vs. infested). Buildup of the SLW population was markedly higher on N‐fertilized plants than on N‐deficient plants. Seed cotton yield was reduced by SLW in festation under all combinations of I × N treatments by about 40%. Yield reduction (noninfested minus infested) was significantly correlated with the number of SLW adults on the maximally infested leaf at peak flowering (r2 = 0.94, P < 0.05 in 1995; r2 = 0.91, P < 0.05 in 1996). Photosynthetic rate decreased 50% at about 60 d after SLW introduction. The relative photosynthetic rate (infested/noninfested) was significantly correlated with SLW adult density on the maximally infested leaf (r2 = 0.72, P < 0.001 in 1995; r2 = 0.53, P < 0.01 in 1996). The reduced photosynthetic rate was associated with reductions in chlorophyll variable fluorescence (Fv/Fm) and fluorescence yield. No substantial changes were found in stomatal conductance, intercellular CO2 concentration, and leaf chlorophyll content. These findings indicate that SLW infestation impairs, either directly or indirectly, the photochemical reaction of the photosynthetic system in cotton plants.

Silverleaf whitefly (SLW), Bemisia argentifolii Bellows and cose and fructose and decreased starch concentrations. Ex-Perring, is one of the most noxious pests of numerous field port rate was determined after 14 CO 2 pulse-labeling both by in situ monitoring of leaf radioactivity and by analyzing the and vegetable crops, causing billions of dollars worth of content and radioactivity of the major carbon metabolites. damage throughout the world. SLW is a phloem feeder Radioactive counting indicated a lower rate of 14 C efflux for whose feeding is likely to interfere with phloem transport. the infested plants. A similar trend was found for the specific The aim of this study was to test the hypothesis that SLW activities of sucrose and the three soluble sugars combined infestation impairs carbohydrate export from source leaves, and consequently increases their carbohydrate content. The (sucrose, glucose and fructose). A single exponential decay youngest fully expanded leaves of cotton (Gossypium hirsu-function with asymptote was fitted to the above efflux curves.All the calculated exponential coefficients demonstrated tum L., cv. Siv'on), grown under SLW-infested and noninfested conditions, were characterized for their diurnal lower export rates after SLW injury. These results indicate changes in carbohydrate content and photoassimilate export. that SLW impairs photoassimilate export, suggesting possible down-regulation of P n due to increased foliar soluble SLW infestation induced a considerable reduction in net sugar contents. photosynthetic rate (P n ), coupled with increased sucrose, glu-

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