Chang-Chi Chu

Stylet Penetration by Bemisia argentifolii (Homoptera: Aleyrodidae) into Host Leaf Tissue

Abstract Silverleaf whitefly, Bemisia argentifolii Bellows & Perring, adults and nymphs feed on the phloem tissue of host plant leaves. Infested leaves were rapidly fixed to prevent the insects from withdrawing their mouthparts. The adult stylet bundle enters the labial groove of the labium between the first and second segments and is completely contained within the labium except during feeding. Stylet length is equal to the combined length of labial segments 2, 3, and 4. Stylet penetration can be determined by examining the position that the stylet enters the labial groove. The physical force necessary for adult stylet penetration is derived from changes in the position of the whitefly head during feeding. The head is bent over the labium, which is attached to the leaf surface, forcing the stylet bundle down the labial groove and into the host tissue. Nymphal stylet lengths were found to be longer than previously reported, and of sufficient length to reach the phloem tissue in cotton and hibiscus leaves from essentially any position on the abaxial leaf surface.

Susceptibility of Upland Cotton Cultivars to Bemisia tabaci Biotype B (Homoptera: Aleyrodidae) in Relation to Leaf Age and Trichome Density

Abstract The relationships between leaf trichome densities, leaf age, and sweetpotato whitefly, Bemisia tabaci (Gennadius) biotype B, infestations of 13 upland cotton, Gossypium hirsutum L., cultivars were investigated in 1990 and 2000. Stoneville 474 supported higher numbers of B. tabaci biotype B eggs, nymphs and adults, and also had higher numbers of stellate trichomes on abaxial leaf surfaces compared with other cotton cultivars. Siokra L-23, in general, had fewer stellate trichomes and also fewer whiteflies. However, the positive trichome–whitefly density relationships were affected by the ages of leaves from different main stem cotton nodes. The youngest leaves on main stem node 1 below the terminal for all cultivars had higher numbers of stellate trichomes but fewer whiteflies compared with older leaves.

Geometric relationships between whitefly feeding behavior and vascular bundle arrangements

This study revealed strong evidence that nymphs of the silverleaf whitefly, Bemisia argentifolii Bellows and Perring, are obligate feeders on vascular bundles and that there are large differences between different host plants as to the availability of vascular bundles to silverleaf whitefly nymphs. The relationship between nymphs and leaf vascular bundles was studied using 1) leaf sectioning and 2) techniques of leaf clearing of intact leaves. A geometric model is presented of the feeding relationship of vascular bundle‐using homopterans. The relative abundance of vascular bundles was examined in six species of host plants that varied from highly preferred to tolerably acceptable. Included in order of acceptance were cantaloupe, cotton, hibiscus, broccoli, lantana and lettuce. The length of vascular bundle per 1.0 mm2 of leaf surface ranged from about 10 mm in cantaloupe to 2.8 mm in lettuce. Salivary sheaths were found to connect with vascular bundles in 100% of the intact nymphs examined by the staining and clearing technique. However only 64% of those examined by the sectioning technique appeared to be connected to vascular bundles. This indicates that the sectioning technique leads to a high rate of error, causing an underestimation of the importance of direct contact with vascular bundles. About 50% of epidermal stylet penetrations were through epidermal cells; the remaining 50% went through intercellular junctions. On cotton leaves, the distance between the point of labial contact with the leaf surface and the nearest point of the vascular bundle rarely exceeded 60 μm. Our studies show that while 50% of lettuce leaf‐surface was beyond 60 μm of a vascular bundle, only 10% of cantaloupe leaf surface area was outside of the 60 μm range. In cotton, mean distance from labium to the nearest point of the vascular bundle was 40.9 μm (SEM = 2.66, N = 50, range 0–80 μm). Over 98% of all salivary sheaths went to minor veins (78% to single‐filament vascular bundles, nearly 20% to double filament bundles). Fewer than 2% went to bundles with 3 or more filaments.

Early Detection and Mass Trapping of Frankliniella occidentalis1, and Thrips tabaci1 in Vegetable Crops

Abstract Western flower thrips, Frankliniella occidentalis (Pergande), and onion thrips, Thrips tabaci Lindeman, are economic pest insects of head lettuce, Lactuca sativa L, and dehydrator onions, Allium cepa L, in the Imperial Valley, California. Colored sticky traps were evaluated as potential detection and monitoring devices of the two thrip species over a two-month period in 2006. Results showed that blue sticky card traps consistently captured more adult thrips of both species compared to yellow sticky card traps. Numbers of thrips captured on blue sticky traps were positively correlated to numbers captured on yellow sticky traps. Adult thrips were captured in much higher numbers on sticky card traps than recovered from whole plant samples. Significant negative correlations between F. occidentalis numbers on plant samples and colored sticky traps suggest that adults left lettuce plants and were attracted to nearby traps In addition, the numbers of thrips caught on traps relative to estimated plant populations support the hypothesis that mass trapping was significant in the treated area. Spectroradiometric and RGB (red, green, blue) digital image color analysis of the traps in sunlight showed the blue sticky cards reflected considerably more light in the 400–500 nm range (R = 49, G = 187, B = 255) than the yellow sticky cards that reflected more light in the 550–700 nm range (R = 227, G = 234, B = 67). Because blue sticky card traps captured many times more adult thrips than recovered from whole plant samples, blue traps may be more sensitive in detecting early presence of thrips in lettuce and onion fields.

Plastic Cup Traps Equipped with Light-Emitting Diodes for Monitoring Adult Bemisia tabaci (Homoptera: Aleyrodidae)

Equipping the standard plastic cup trap, also known as the CC trap, with lime-green light-emitting diodes (LED-plastic cup trap) increased its efficacy for catching Bemisia tabaci by 100%. Few Eretmocerus eremicus Rose and Zolnerowich and Encarsia formosa Gahan were caught in LED-plastic cup traps. The LED-plastic cup traps are less expensive than yellow sticky card traps for monitoring adult whiteflies in greenhouse crop production systems and are more compatible with whitefly parasitoids releases for Bemisia nymph control.

Economic injury levels for Bemisia tabaci (Homoptera: Aleyrodidae) in cotton: impact of crop price, control costs, and efficacy of control

Abstract Field studies were conducted during 1993 and 1994 in the Imperial Valley, California and Maricopa, Arizona to examine relationships between densities of Bemisia tabaci (Gennadius) populations and cotton (Gossypium hirsutum L.) yields, and to estimate economic injury levels (EILs) for pest management application. Populations of B. tabaci were manipulated by applying different numbers of insecticide applications in replicated plots. Resulting insect populations and cotton lint yields were used to develop damage functions and to estimate EILs for all life stages in relation to variable cotton prices, insecticide costs, and pest control efficacy. Economic injury levels declined with increasing cotton prices and increased as the cost of control increased, however, these changes were relatively small, based on typical ranges in price and control costs. In contrast, the efficacy of control provided by insecticide applications had a large influence on EILs, with lower efficacies being associated with higher injury levels. We developed a multiple regression model that accounted for the dynamic changes in the EIL in relation to crop price, control costs, control efficacy, and potential yield. Based on average prices and reasonable control costs and efficacy, EILs ranged from 5.9–15.2 adults/leaf, 6.1–19.8 eggs cm−2, and 1.7–4.7 nymphs cm−2 of leaf area. Additional research is needed to more closely relate the costs of control to the suppression of insect populations, and to define economic thresholds that will enable pest managers to maintain pest populations below EILs.

Trap Evaluations for Thrips (Thysanoptera: Thripidae) and Hoverflies (Diptera: Syrphidae)

Abstract Various trap types were evaluated for catching western flower thrips, Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae), and hoverflies (Diptera: Syrphidae). More western flower thrips were attracted to blue (458-nm peak reflectance) sticky card traps compared with yellow (560 nm) or white sticky card traps. Blue light-emitting diodes (LEDs; 465-nm peak emission) increased the efficacy of blue sticky card traps for catching western flower thrips. Numbers of western flower thrips caught on blue flat rectangular card traps were increased 2.0–2.5 times when cards were equipped with blue LEDs. Hoverflies adults are pollinators, and larvae are natural enemies of aphids and other pest insects. Hoverfly adults were attracted to blue sticky card traps. Nylon screen cages effectively excluded the hoverflies from the blue sticky card traps.

DEVELOPING AND EVALUATING TRAPS FOR MONITORING SCIRTOTHRIPS DORSALIS (THYSANOPTERA: THRIPIDAE)

Abstract Scirtothrips dorsalis (Hood) (Thysanoptera: Thripidae) is a recently identified invasive pest to the Caribbean and poses a significant threat to agriculture and trade in the region. Methods are needed to detect the presence and to monitor populations of this pest so that it can be effectively managed. Three different CC trap base colors (blue, yellow, and white) with or without dichlorvos as a killing agent, and a newly developed and named the Blue-D trap were studied in Taiwan and St. Vincent for attraction and capture of S. dorsalis. In lemons in Taiwan, mean numbers of S. dorsalis caught in Blue-D traps were greater compared with dichlorvos cube modified CC traps. In St. Vincent chili pepper plantings, the Blue-D traps caught more Thrips palmi (Karny), Frankliniella sp., and Microcephalothrips abdominalis (Crawford) than dichlorvos cube modified CC traps. More Frankliniella intonsa (Trybom), Megalurothrips usitatus (Bagnall), T. palmi, Frankliniella sp., and M. abdominalis were caught in blue and white base CC traps than yellow base CC traps. Average captures per CC trap per week were 0.07 and 0.02-0.09 S. dorsalis in Taiwan and St. Vincent, respectively. There were no differences in S. dorsalis captures in white, blue, or yellow base CC traps. The average weekly S. dorsalis catch for yellow sticky card traps was 19.8. CC traps can be used for detection of S. dorsali and collecting intact S. dorsalis for taxonomic and genetic determinations when a few of the species are found in a large commercial production area. Yellow sticky traps can be used for monitoring S. dorsalis populations. A combination detecting system of visual observation, yellow sticky traps, and CC traps may be an effective S. dorsalis population detecting and monitoring system.

Bemisia tabaci (Homoptera: Aleyrodidae) Biotype B Colonization on Okra- and Normal-Leaf Upland Cotton Strains and Cultivars

Abstract We compared smooth-leaf okra- and normal-leaf upland cotton (Gossypium hirsutum L.) strains and cultivars for susceptibility to colonization by Bemisia tabaci (Gennadius) biotype B. Experiments were conducted at seven field sites, five at Holtville, CA, and two at Maricopa, AZ, during 1996–2000. Okra-leaf strains and cultivars, as a group, had lower numbers of adults, eggs, and nymphs compared with normal-leaf strains and cultivars indicating the potential of okra-leaf genetic traits for reducing colonization by B. tabaci. Results also suggest that okra-leaf shape may provide less favorable micro-environmental conditions for the habitat of B. tabaci because of more open canopy as evidenced by higher leaf perimeter to leaf area ratio. The okra-leaf cultivar ‘Siokra L-23’ appears to have genetic traits that should be examined further as a source of B. tabaci resistance.

Bemisia argentifolii (Homoptera: Aleyrodidae) Colonization on Upland Cottons and Relationships to Leaf Morphology and Leaf Age

Abstract We compared Bemisia argentifolii Bellows & Perring colonization on Stoneville (ST) 474 and Deltapine (DPL) 5415 cottons, Gossypium hirsutum L., in the field. We also examined leaf trichome density, leaf age, and leaf morphological characteristics as possible factors influencing cultivar host selection. The increased numbers of all B. argentifolii life stages on ST 474 in the field appeared to be related to the higher trichome density on abaxial leaf surfaces compared with DPL 5415. In both cultivars, leaves from node number 1 below the terminals were smaller and had higher vascular bundle densities and numbers of lysigenous glands than older, larger leaves. Younger leaves also had smaller leaf areole areas, more terminal vein endings per unit leaf area, and shorter distances from abaxial leaf surfaces to minor vein phloem tissues compared with older leaves. These younger leaf morphological characteristics may contribute to the higher B. argentifolii densities on younger leaves. In the laboratory, electronically monitored adult females and visually monitored settled first and fourth instars preferred to probe into secondary and tertiary leaf veins as compared with main and primary leaf veins.