David N. Byrne

Carbohydrate and amino acid composition of phloem sap and honeydew produced by Bemisia tabaci

Abstract A Florida strain of sweet potato whitefly, Bemisia tabaci (Gennadius), was found to have an expanded range which includes several new food crops. To determine why, we examined how it processes plant nutrients. The amino acid and carbohydrate content of phloem sap from poinsettia and pumpkin and of honeydew produced by the Florida strain were analysed. Honeydews produced by a strain from Arizona feeding on both plants were also analysed. Poinsettia phloem sap contained 15 amino acids; 14 of these were in pumpkin phloem sap. Almost all the same amino acids were in the honeydews produced by the two strains on the two hosts. Approximately half of the amino acids found in the honeydew were at concentrations which were significantly lower than concentrations in the phloem sap. Honeydew from both hosts contained six additional amino acids. The major one was glutanine which may be used to expel nitrogen. Carbohydrates in phloem sap and honeydew were common transport sugars, like sucrose. Both honeydews contained trehalulose, a disaccharide not previously associated with insects. Both strains processed phloem sap and honeydew from both plants in the same manner, but the Florida strain produced significantly larger quantities of honeydew; it is therefore assumed to process more phloem sap. Since this strain has access to more phloem sap it also has access to more of the amino acids which are in short supply in the phloem sap of some plants allowing it to broaden its range.

The effects of crop diversification on herbivorous insects: a meta‐analysis approach

Abstract. 1 We used a meta‐analysis to evaluate the hypothesis that diversified crops are subject to lower density of herbivorous insects. This method consists of integrating findings of independent studies by calculating the magnitude of treatment effects (effect size). 2 A literature review covering a period of over 10 years yielded twenty‐one studies that contained sufficient information for the meta‐analysis. The data were analysed according to three criteria: yearly replications of the same study were considered independent data so that research repeated over time would have greater weight; a second meta‐analysis was conducted with results of only 1 year because of possible lack of independence between results of the same study; the third approach was to remove studies involving Brassica spp. from the analysis because they are redundant from the standpoint of testing a general hypothesis of crop diversification. 3 The three analyses resulted in effect sizes of 0.50, 0.35 and 0.27, respectively, and all were statistically significant. These results correspond to 69%, 63% and 60% differences between average insect densities for treatment (diversified crops) and control groups. 4 We considered the last criterion for data analysis as the most appropriate because of lowest bias from non‐independence between samples. Therefore we concluded that crop diversity caused moderate reduction of herbivorous insect populations. 5 The implications and limitations of meta‐analyses are discussed.

Localized migration and dispersal by the sweet potato whitefly, Bemisia tabaci

Laboratory populations of the sweet potato whitefly, Bemisia tabaci, have been shown to consist of both migratory and trivial flying morphs. The behavior of these forms as part of the process of short-range migration was examined under field conditions. Insects were marked in a field of melons using fluorescent dust during two consecutive growing seasons. During the first growing season, passive traps used to collect living whiteflies, were placed along 16 equally spaced transects radiating from the field to a distance of up to 1.0 km. Wind out of the north-east consistently carried migrating whiteflies to traps placed along transects in the south-western quadrant because cold air drainages dictate wind direction during early morning hours in the desert South-west. For this reason, during the second season traps were laid out over fallow ground in a rectangular grid extending 2.7 km to the south-west of the marked field. If dispersal was entirely passive, patterns could be described using a diffusion model. Statistical examination of the data, howèver, demonstrated that the distribution on all days was patchy. Geostatistical techniques were used to describe the observed patchiness. Traps in the immediate vicinity of the marked field caught more whiteflies than the daily median. Large numbers were also collected from near the periphery of the grid. White-flies were far less prevalent in the grids center. As a result, the distribution of captured whiteflies can be described as bimodal. These patterns confirm behavior observed in the laboratory, i.e., a portion of the population are trivial fliers that do not engage in migration and are consequently captured in traps near the field, and a portion initially respond to cues associated with skylight, ignoring cues provided by the ground, and fly for a period of time before landing in distant traps. During both years movement out of the field had an exaggerated directional component on 13 of 14 days.

Flight behaviour of Bemisia tabaci in a vertical flight chamber: effect of time of day, sex, age and host quality

Abstract. The free‐flight behaviour of Bemisia tabaci (Gennadius) (Homoptera: Aleyrodidae), the sweet potato whitefly, was investigated in a vertical flight chamber. A mercury‐vapour lamp presented from above induced a phototactic flight response. Although flight propensity was comparable from 06.00 to 19.00 hours, flight duration was maximum between 06.00 and 10.00 hours. Males flew longer than females and their mean flight duration remained constant throughout the day. Females flew longer from 06.00 to 13.00 hours than from 13.00 to 19.00 hours. Both sexes were capable of sustaining flight for more than 2h, although less than 5% of those tested did so.

Effects of gender and mating status on self‐directed dispersal by the whitefly parasitoid Eretmocerus eremicus

1. The impacts of gender and mating on short‐range (< 10 km) dispersal by the whitefly parasitoid Eretmocerus eremicus Rose and Zolnerowich (Hymenoptera: Aphelinidae) were examined. A fluorescent dust‐marking technique was also tested.

Presence of trehalulose and other oligosaccharides in hemipteran honeydew, particularly Aleyrodidae

Abstract. Production by a bacterium of the disaccharide trehalulose was reported 30 years ago. The association between the fabrication of trehalulose and Insecta was found more recently. It was initially discovered in the honeydew, excreta, of the whitefly Bemisia tabaci feeding on Euphorbia pulcherrima where it was the predominant sugar. In the present study, B. tabaci‐produced trehalulose was again found in honeydew at significant levels, this time when the whitefly fed on 10 different plants. In seven of 10 Bemisia/host combinations, trehalulose accounted for more than 30% of the total carbohydrates found in their honeydew and was the principal oligosaccharide. Trehalulose constituents, glucose and fructose, were also present at lower levels, as were other oligosaccharides such as melezitose. Feeding by B. tabaci on the three other plant hosts also resulted in the production of trehalulose at relatively high levels (6.1–16.5%). Other whitefly species examined had little (e.g. B. afer and Aleurothrixus spp.), or no trehalulose (e.g. Trialeurodes spp. and Siphoninus phillyreae), in their honeydew. Trehalulose was also found in the honeydew of two aphid and one scale insect species. In insects whose honeydew had low levels or no trehalulose, sucrose, its constituents, or larger sugars predominated. The trisaccharide bemisiose was also found in the honeydew of half the whitefly species examined. Bemisiose was discovered here for the first time in the honeydew of three aphid species and three species of scale insects. Reasons for the production of trehalulose are often linked to high levels of dietary sucrose in whiteflies. This is probably true in our case because cotton and cucurbits, at least, are known to contain a great deal of sucrose. Although other functional possibilities were explored, the data suggest that trehalulose, being less susceptible to hydrolysis than sucrose, is involved in osmoregulation.

Development of a monoclonal antibody to detect predation of the sweetpotato whitefly, Bemisia tabaci

We have developed an enzyme‐linked immunosorbent assay (ELISA) for detection of whitefly antigen in the guts of predaceous arthropods based on a monoclonal antibody (MAb) produced in response to Bemisia tabaci (Gennadius) (Homoptera: Aleyrodidae) egg antigen. We observed positive antigen‐antibody reactions with the egg and the adult female stages of certain whitefly species and not with others. This MAb did not cross‐react with any of the whitefly nymphs or adult males we tested. Moreover, this MAb did not cross‐react with the various life stages of other insect species we examined. This immunologically‐based technique to identify whitefly egg facilitates examinations of predator gut contents in the field. Such a specific and sensitive whitefly egg immunoassay will expedite the characterization of the B. tabaci predator complex in the southwestern United States.

Changes in amino acids in Cucumis melo in relation to life‐history traits and flight propensity of Bemisia tabaci

Phloem amino acids in Cucumis melo L. were measured to determine whether changes in nitrogen availability might affect life‐history traits and flight activity of Bemisia tabaci Gennadius (Homoptera: Aleyrodidae). During plant development, nineteen of the twenty common amino acids, plus hydroxyproline, citrulline, ornithine and γ‐aminobutyric acid (GABA) were identified. For most essential amino acids, there were two peaks observed: an initial large peak associated with young plants, and a later small peak associated with senescing plants. For histidine, ornithine and citrulline, medium to large peaks in relative abundance were observed in mature plants. Arginine peaked during the first few weeks of development and was no longer detectable after wk 7. Serine and glutamine/glutamic acid were the only amino acids that peaked during plant senescence. Factor analysis was used to create a reduced number of orthogonal factors, which corresponded well with the trends that were observed for the various groups of amino acids. No single or combination of factors explained a significant amount of the variability in oviposition. For both males and females, factor 1 (predominantly essential amino acids) was the single most important predictor of adult weight. As the relative concentrations of essential amino acids decreased, whitefly weights decreased. Factors 1 and 3 (predominantly histidine and ornithine) were the most important predictors of developmental time. As these amino acids decreased in relative concentration, developmental time increased. Percent emergence was positively associated with factor 1 and negatively associated with factor 6 (predominantly aspartic acid). The distributions of flight duration for whiteflies emerging from young, mature and senescing melon plants were compared and they were always skewed towards short flights; however, the frequency of long‐duration flights increased when melon plants began to senesce. Whiteflies from all plant‐age categories were capable of flying for more than 2 h with fully developed eggs, but the presence of more than four mature eggs was associated with flights of reduced duration.

FEEDING RATES AND CARBOHYDRATE METABOLISM BY BEMISIA TABACI (HOMOPTERA : ALEYRODIDAE) ON DIFFERENT QUALITY PHLOEM SAPS

Abstract.The effects of water stress on phloem sap quality of the melon, Cucumis melo, and how this, in turn, has an impact on the sweet potato whitefly, Bemisia tabaci were studied. Melon plants were grown under watering regimes that produced plants with or without water stress. Plants showed strong developmental responses to the treatments; water‐stressed plants were shorter, with fewer, smaller leaves than those without stress. There was, however, no effect of plant water stress on the development period of whiteflies feeding on these plants, or on the weights of male or female adults. Honeydew production was used as an indirect measure to test whether the absence of insect developmental or behavioural effects was due to differential phloem sap ingestion. Feeding rates on the stressed plants were almost half those on unstressed plants, and there was also variation in the daily pattern of honeydew production. Phloem sap and honeydew were analysed to determine why the feeding behaviours differed. Amino acid composition of the phloem sap was similar in both groups of plants, but carbohydrate concentrations were greater in water‐stressed plants, indicating that lower feeding rates may have been due either to the physical or nutritional quality of the phloem sap. The honeydew of insects that were feeding on water‐stressed plants contained a greater concentration of carbohydrate than those on unstressed plants, and was composed of a significantly greater proportion of glucose and the disaccharide, trehalulose. This isomerization of more complex sugars from those in the diet suggests that B. tabaci uses a mechanism of osmoregulation to actively maintain its internal water status. It is concluded that transient conditions of water stress in this host plant do not affect the development of B. tabaci, due to physiological and behavioural changes in response to diets with different nutritional and physical properties. The implications of this finding for the feeding biology of B. tabaci on desert‐grown crops are discussed.

Behavioral, morphological, and physiological traits associated with migratoryBemisia tabaci (Homoptera: Aleyrodidae)

Bemisia tabaci, that were flown for varying lengths of time (1 min-2.5 h), exhibited behavioral, morphological, and physiological differences. When whiteflies that were engaging in phototactic orientation in a vertical flight chamber were presented with a visual cue simulating their host plant (550-nm narrowband interference filter), 76% landed on this cue within three presentations. Another 18% of the whiteflies displayed an intermittent attraction to the host cue, and 6% displayed a response that, historically, has been considered to be indicative of migration. These individuals failed to respond to the target until they had flown for at least 15 min. These three categories of response were observed in both sexes, in all flight-capable individuals from 1 to 5 days old and in two groups of whiteflies that were exhibiting distinct behaviors prior to our tests (i.e., they were either settled on poinsettia or engaging in phototactic orientation). There was a trend for males that engaged in long-distance flight to have smaller wing dimensions than males that engaged in shorter flights; no trend was evident in female whiteflies. High levels of vitellogenin and vitellin did not inhibit flight activity. In fact, we generally found higher levels of egg proteins in long-distance fliers and in premigratory individuals than in short-duration fliers and settled individuals.