Eric T. Natwick

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.

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.

A New Expanded Host Range of Cucurbit yellow stunting disorder virus Includes Three Agricultural Crops

Cucurbit yellow stunting disorder virus (CYSDV) was identified in the fall of 2006 affecting cucurbit production in the southwestern United States (California, Arizona), as well as in nearby Sonora, Mexico, resulting in nearly universal infection of fall melon crops in 2006 and 2007, and late infection of 2007 spring melons. Survival of CYSDV through the largely cucurbit-free winter months suggested the presence of weed or alternate crop hosts, although previous studies indicated a limited host range restricted to members of the Cucurbitaceae. To determine potential reservoir hosts for CYSDV in desert production, weed and crop hosts were collected from throughout the region over a period of 26 months, and were tested for the presence of CYSDV by reverse transcription-polymerase chain reaction (RT-PCR) using CYSDV HSP70h- and coat protein gene-specific primers. Many noncucurbits collected from infected melon fields and nearby areas were symptomless and virus free; however, CYSDV was detected in alfalfa (Medicago sativa), lettuce (Lactuca sativa), and snap bean (Phaseolus vulgaris), as well as in several weed species widely prevalent in the region. Typical crinivirus symptoms of interveinal yellowing and leaf brittleness were observed on CYSDV-infected snap bean, alkali mallow (Sida hederacea) and Wrights groundcherry (Physalis wrightii), while other infected crop and weed hosts were symptomless. Transmission tests demonstrated that lettuce, snap bean, alkali mallow, Wrights groundcherry, and buffalo gourd (Cucurbita foetidissima) could serve as virus reservoir hosts for transmission of CYSDV to melon and other cucurbits. These results expand the previously known host range of CYSDV, demonstrating that the virus is capable of infecting not only members of the Cucurbitaceae, but also plants in seven additional taxonomic families.

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.

Integrated Systems for Managing Bemisia tabaci in Protected and Open Field Agriculture

The combined efforts of disparate entities have produced notable advances in management of Bemisia tabaci (Gennadius) over the decades. On the one hand, entomologists and academicians have focused on specific problems, approaches and solutions, and designed experiments with replicated treatments to provide statistically valid results that hold up to scientific scrutiny. On the other hand, growers must integrate information from all disciplines into a profitable cropping and marketing system in order to survive. The gap between these extremes is often filled by crop consultants or agrochemical sales representatives who may have limited interests, focus or experience. There is clearly a need for more and better information on how management practices can be integrated to provide the desired level of pest suppression while still maintaining a balanced and profitable cropping system. The key challenge to implementing new practices will be to sustain or improve current levels of productivity while minimizing impact on the environment and biodiversity.

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.

Host-Specific Relationship Between Virus Titer and Whitefly Transmission of Cucurbit yellow stunting disorder virus

Cucurbit yellow stunting disorder virus (CYSDV; genus Crinivirus, family Closteroviridae) was identified in the melon (Cucumis melo) production regions of the desert southwestern United States in fall 2006. It is now well established in the region, where it is transmitted efficiently by the sweet potato whitefly, Bemisia tabaci biotype B (MEAM1). In order to evaluate the spread and establishment of the virus, nearly all spring and fall cucurbit fields planted in the Imperial Valley of California from 2007 to 2009 were surveyed and representative plants were tested for CYSDV infection. Incidence of CYSDV in spring melon fields was initially low and limited to a small number of fields in 2007 but increased to 63% of fields by spring 2009. Virus incidence in fall melon fields was 100% in each year. These results suggested that the virus had become established in native vegetation, weeds, and other crop species, and represented an increasing threat to melon production in the southwestern United States. Therefore, a select set of weed and crop species which grow or are cultivated in the Imperial Valley were evaluated as CYSDV reservoir hosts. For each species, we determined the capacity of CYSDV to accumulate, the relationship between virus titer in these source plants and transmission by whiteflies, as well as subsequent accumulation in inoculated cucurbit plants. Among these hosts, there was considerable variation in virus accumulation and transmission rates. Cucurbit hosts had the highest CYSDV titers, were efficient sources for virus acquisition, and showed a positive correlation between titer in source plants and transmission. Noncucurbit hosts had significantly lower CYSDV titers and varied in their capacity to serve as sources for transmission. CYSDV titers in some noncucurbit source plants, specifically common bean (Phaseolus vulgaris) and shepherds purse (Capsella bursa-pastoris), were not positively correlated with transmission, demonstrating that additional environmental, physical, or biochemical factors were involved. These results demonstrate that multiple factors influence the efficiency with which a host plant species will be a reservoir for vector transmission of virus to crops.

Research Reports: Protection From Phytophagous Insects and Virus Vectors in Honeydew Melons Using Row Covers

Protection of Honeydew melons, Cucumis melo, from infestation by the sweetpotato whitefly, Bemisia tabaci, (Gennadius) and the vegetable leafminer, Liriomyza sativae Blanchard, varied relative to the row cover product used. Melon plants were also protected in varying degrees from squash leafcurl virus and lettuce infectious yellows virus. Exclusion of sweetpotato whitefly was best achieved by Kimberly Farms row cover followed by Reemay, LF0125, Tufbel 900N, Tufbel 4000N, and Agronet. All row cover treatments had fewer insect pests and were lower in disease incidence than the untreated control. Vegetable leafminer was completely excluded by all row cover products. The number of honeydew melons harvested from Kimberly Farms treated plots was greatest followed by Tufbel 900N, Agronet, LF0125, Reemay, Tufbel 4000N and the untreated control.

Trap catches of the sweetpotato whitefly (Homoptera: Aleyrodidae) in the Imperial Valley, California, from 1996 to 2002

An outbreak of the sweetpotato whitefly, Bemisia tabaci (Gennadius), biotype B occurred in the Imperial Valley, California in 1991. The insects destroyed melon crops and seriously damaged other vegetables, ornamentals and row crops. As a result of the need for sampling technology, we developed a whitefly trap (named the CC trap) that could be left in the field for extended time periods. We used the traps to monitor populations of B. tabaci adults during year‐round samplings from 1996 to 2002 to study variations in the weekly trap catches of the insect. The greatest number of B. tabaci adults was recorded in 1996, followed by a continuing annual decrease in trap catches each year through 2002. The overall decline of B. tabaci is attributed in part to the adoption of an integrated pest management (IPM) program initiated in 1992 and reduced melon hectares from 1996 to 2002. Other factors may also have contributed to the population reductions. Seasonally, B. tabaci trap catches decreased during the late summer and fall concurrent with decreasing minimum temperatures that are suggested to be a significant factor affecting seasonal activity and reproduction.

Pink bollworm moth (Lepidoptera: Gelechiidae) catches in the Imperial Valley, California from 1989 to 2003

Abstract We examined the patterns of male pink bollworm (PBW), Pectinophora gossypiella (Saunders), moth catches in gossyplure‐baited traps over a 15‐year period from 1989 to 2003 in the Imperial Valley, California, USA. Monitoring was conducted during periods when different pink bollworm areawide control strategies were being used. Numbers of male pink bollworm moths caught in gossyplure‐baited traps progressively decreased each year from 1990 to 1994 during short‐season cotton production. High numbers of male moths caught in traps from 1995 to 1997 may have been related to moth migrations from the large cotton acreages grown in the Mexicali Valley bordering the Imperial Valley. Transgenic Bollgard® (Bt) cotton was planted in 3% of the cotton area in 1996 and thereafter in 80%–94% of the cotton area from 1997 to 2003. Pink bollworm moth trap catches were significantly lower from 1998 to 2003 than catches in 1995 to 1997, except for 1999. The trapping results suggested that Bt cotton had significant input on reduction of pink bollworm populations, confirming results of other investigators and providing additional documentation on the benefits of the Bt cotton culture.