Thomas M. Perring

The Bemisia tabaci species complex

Abstract Bemisia tabaci (Gennadius) currently is believed to be a species complex, with many biotypes and two described extant cryptic species, B. tabaci and Bemisia argentifolii Bellows and Perring. This paper examines the historical record of B. tabaci and its synonyms, and discusses research that has resulted in the species complex hypothesis. It also reviews work that has been done comparing populations from various geographic locations. This comparative research, using a variety of techniques, forms the basis from which similarities and differences between populations can be inferred. Based on these studies, seven groups within the species complex are proposed.

Local and Systemic Changes in Squash Gene Expression in Response to Silverleaf Whitefly Feeding

Squash genes (SLW1 and SLW3) induced systemically after silverleaf whitefly feeding were identified. Differences in the local and systemic expression of SLW1 and SLW3 after feeding by the closely related silverleaf and sweetpotato whiteflies were observed. Temporal and spatial studies showed that SLW1 and SLW3 were induced when second, third, and fourth nymphal instars were feeding. Although only barely detected after wounding and bacterial infection, SLW1 and SLW3 RNAs were abundant during water-deficit stress. Treatments with wound/defense signal molecules showed that SLW1 RNAs accumulated in response to methyl jasmonate and ethylene, whereas SLW3 was not regulated by known wound/defense signals, suggesting utilization of a novel mechanism for defense signal transduction. SLW1 RNAs accumulated during floral and fruit development, whereas SLW3 RNAs were not detected during vegetative or reproductive development. The potential roles of SLW1, an M20b peptidase–like protein, and SLW3, a β-glucosidase–like protein, in defense and the leaf-silvering disorder are discussed.

Plant virus-induced changes in aphid population development and temporal fluctuations in plant nutrients.

Cucurbita pepo plants were infected with zucchini yellow mosaic virus or maintained noninfected.Aphis gossypii, which transmits the virus, lived longer and produced more offspring on infected than on noninfected plants. On infected plants, the intrinsic rate of natural increase forA. gossypii increased with time after inoculation. In a similar experiment, concentrations of phloem sap nutrients, including free amino acids, total protein, and sugars from infected and noninfected plants were compared for 37 days after inoculation. Significant differences in levels of individual amino acids from phloem exudate between infected and noninfected plants were found, yet the concentration of total amino acids was not substantially different between infected and noninfected plants. Beginning four days after inoculation, the total protein content of phloem exudate generally was lower in infected plants than noninfected plants. Likewise, the total sugar content of phloem exudate from infected plants was lower than that of noninfected plants beginning nine days after inoculation. In contrast with the results from analyses of phloem exudate, foliage from infected plants had higher levels of almost all amino acids than noninfected foliage beginning nine days after inoculation. Concentrations of individual and total amino acids in infected foliage increased throughout the experimental period. Although no temporal effects were observed in the foliage sugar content for either individual or total sugars, starch content decreased with time in infected plants, while in noninfected plants, starch content remained level.

Genetic Analysis of Bemisia (Hemiptera: Aleyrodidae) Populations by Isoelectric Focusing Electrophoresis

Twenty-one whitefly populations in the genus Bemisia were evaluated for genetic variation at 3 allozyme loci. Nine of the 22 populations that exhibited polymorphic loci were subjected to allozyme analysis using a minimum of 10 enzymes, representing 10 to 14 distinct loci. Among those nine variants examined, calculated genetic distances ranged between 0.03 and 0.52, with three main groups emerging from the analysis. One group comprised two closely related Western Hemisphere variants of B. tabaci: type A from California, United States and a geographically proximal population from Culiacan, Mexico. A second cluster contained five collections previously identified as B. tabaci type B and Bemisia argentifolii, while a third group contained a single population from Benin, Africa. The latter two groups were grouped separately from New World populations and are thought to have a recent origin in the Eastern Hemisphere.

Alatae production and population increase of aphid vectors on virus-infected host plants

SummaryZucchini yellow mosaic virus (ZYMV) and Aphis gossypii Glover are two components of a recently identified plant-parasite system that provides an excellent opportunity to study interrelations between a virus and a vector that share the same host, but have no direct physiological interaction. In a field experiment we documented numbers of alate and apterous A. gossypii on healthy Cucurbita pepo and on plants inoculated with virus 0, 7, 14, and 21 days before aphid infestation. When plants were inoculated and infested simultaneously, more than twice as many alatae were produced after 20 days of colony growth than on any other treatment. This indicates that properties unique to the early stages of viral infection somehow stimulated wing formation. Because it is spread by the activities of alatae, virus dispersal would be greater as a result of these properties. Developmental rate, total numbers of aphids, and numbers of alatae and apterae decreased as the time between virus inoculation and aphid colonization increased.

Tomato Pathogenesis-related Protein Genes are Expressed in Response to Trialeurodes vaporariorum and Bemisia tabaci Biotype B Feeding

The temporal and spatial expression of tomato wound- and defense-response genes to Bemisia tabaci biotype B (the silverleaf whitefly) and Trialeurodes vaporariorum (the greenhouse whitefly) feeding were characterized. Both species of whiteflies evoked similar changes in tomato gene expression. The levels of RNAs for the methyl jasmonic acid (MeJA)- or ethylene-regulated genes that encode the basic β-1,3-glucanase (GluB), basic chitinase (Chi9), and Pathogenesis-related protein-1 (PR-1) were monitored. GluB and Chi9 RNAs were abundant in infested leaves from the time nymphs initiated feeding (day 5). In addition, GluB RNAs accumulated in apical non-infested leaves. PR-1 RNAs also accumulated after whitefly feeding. In contrast, the ethylene- and salicylic acid (SA)-regulated Chi3 and PR-4 genes had RNAs that accumulated at low levels and GluAC RNAs that were undetectable in whitefly-infested tomato leaves. The changes in Phenylalanine ammonia lyase5 (PAL5) were variable; in some, but not all infestations, PAL5 RNAs increased in response to whitefly feeding. PAL5 RNA levels increased in response to MeJA, ethylene, and abscisic acid, and declined in response to SA. Transcripts from the wound-response genes, leucine aminopeptidase (LapA1) and proteinase inhibitor 2 (pin2), were not detected following whitefly feeding. Furthermore, whitefly infestation of transgenic LapA1:GUS tomato plants showed that whitefly feeding did not activate the LapA1 promoter, although crushing of the leaf lamina increased GUS activity up to 40 fold. These studies indicate that tomato plants perceive B. tabaci and T. vaporariorum in a manner similar to baterical pathogens and distinct from tissue-damaging insects.

Courtship Behavior of Bemisia argentifolii (Hemiptera: Aleyrodidae) and Whitefly Mate Recognition

Abstract Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) consists of a species complex with various degrees of reproductive compatibility between biotypes. One mechanism known to result in reproductive isolation among sexually reproducing animals is mate recognition. Whiteflies have an elaborate courtship and mating behavior, and it is well known that individuals of some biotypes in the B. tabaci species complex will court individuals of other biotypes, but they will not mate. In this study, we determined specific courtship and mating behaviors of B. tabaci biotype B (Bemisia argentifolii Bellows & Perring). Four distinct phases in the courtship and mating cascade were identified, and we describe these phases and report their durations. We compared our findings with previously reported mating behaviors of two other whiteflies, B. tabaci biotype A, and Trialeurodes vaporariorum (Westwood). These comparisons identified both similarities and differences in the behaviors of the three whiteflies, particularly in the extent and position of antennal drumming, male abdominal undulation, and wing and body position during copulation. Body pushing behavior, characterized for B. tabaci biotype A and T. vaporariorum, was not present for B. argentifolii. The similarities between whiteflies may represent evolutionarily conserved behaviors, resulting in courtships between reproductively incompatible whiteflies. Conversely, differences in behaviors may contribute to prezygotic reproductive isolation among biotypes. From our studies, we propose that the discrimination of signals sent and received from courting whiteflies becomes more intense with each successive phase in the courtship cascade.

Effects of microenvironment on the dynamics of spider-mite populations

The relationship between environmental variables (chiefly temperature and humidity) and the population dynamics of spider mites is reviewed. Both direct effects on the spider mites and indirect effects operating through effects on spider mite natural enemies (mainly phytoseiid mites) are discussed. Factors determining the environmental conditions actually experienced by spider mites (microenvironment) are presented.Microenvironmental information versus environmental information from nearby weather stations is evaluated for utility in predicting spider mite population dynamics. A comprehensive plant canopy/spider mite/phytoseiid model is used to simulate an irrigated maize/spider-mite/phytoseiid system in a semi-arid climate. Under nearly all tested combinations of weather and irrigation, substantial differences were seen between simulations that considered microenvironment and those that considered only environmental conditions above the plant canopy. Future research needs are discussed.

Cross protection of cantaloupe with a mild strain of zucchini yellow mosaic virus: effectiveness and application

Abstract Plants infected with a severe strain of zucchini yellow mosaic virus (ZYMV-CA) produced fewer marketable fruit than non-infected control plants or plants infected with a mild strain of ZYMV (ZYMV-WK); cross protection conserved marketable yield by nearly 75%. Cantaloupe plants in the non-infected control treatment and plants infected solely with ZYMV-WK produced similar numbers of marketable fruit. In another study, ZYMV-WK was introduced successfully into cantaloupe plants using standard agricultural spray equipment at 2.1 kg cm −2 pressure. Infection with the mild strain was related directly to the concentration of virus in our sprays. The highest infection rate (77%) was achieved with a 10% solution of fresh tissue in buffer. Lower percentage solutions, 2 and 1%, resulted in 27.5 and 25% infection, respectively. A 10% spray solution made from frozen infected tissue resulted in 56% infection. Our studies demonstrate the effectiveness of cross protection in cantaloupe and offer a method to growers for applying this technology to large-scale, direct-seeded agriculture.

Feeding habits of the Tydeidae with evidence of Homeopronematus anconai (Acari: Tydeidae) predation on Aculops lycopersici (Acari: Eriophyidae)

Abstract A literature review of the Tydeidae indicated that feeding habits include predation, phytophagy, fungivory, and scavenging. In the current study, evidence for predation of Homeopronematus anconai (Baker) on Aculops lycopersici (Massee) is presented. The tydeid was able to reproduce after feeding on various life stages of the eriophyid mite. In laboratory experiments, significantly lower population densities of A. lycopersici developed on individual tomato leaflets and seedlings that had H. anconai present, compared to leaflets and seedlings void of H. anconai. In addition, H. anconai phytophagy (on pollen and leaf tissue) and fungivory (on Cladosporium cladosporoides) were observed.