In 1991, the poinsettia strain, silverleaf whitefly or B biotype of Bemisia tabaci was detected in Brazil. This variant is a far more serious agricultural pest than the previously prevalent non-B (BR) biotype. The correct identification of B. tabaci is problematic since it is highly polymorphic with extreme plasticity in key morphological characters that vary according to the host. RAPD-PCR was used to survey the B biotype and other biotypes of B. tabaci in Brazil. Whiteflies were collected from cultivated plants and weeds from 57 different localities and on 27 distinct crops. RAPD analyses using two selected 10-mer primers reliably identified the BR biotype and the B biotype of B. tabaci and also differentiated other whitefly species. The presence of the B biotype was confirmed in 20 Brazilian states. The BR and B biotypes of B. tabaci were found to coexist in the whitefly populations of three different localities: Jaboticabal, SP; Rondonópolis and Cuiabá, MT, and Goiânia, GO.

Bemisia tabaci (Genn.) was considered a secondary pest in Brazil until 1990, despite being an efficient geminivirus vector in beans and soybean. In 1991, a new biotype, known as B. tabaci B biotype (=B. argentifolii) was detected attacking weed plants and causing phytotoxic problems in Cucurbitaceae. Nowadays, B. tabaci is considered one of the most damaging whitefly pests in agricultural systems worldwide that transmits more than 60 different plant viruses. Little is known about the genetic variability of these populations in Brazil. Knowledge of the genetic variation within whitefly populations is necessary for their efficient control and management. The objectives of the present study were to use RAPD markers (1) to estimate the genetic diversity of B. tabaci populations, (2) to study the genetic relationships among B. tabaci biotypes and two other whitefly species and (3) to discriminate between B. tabaci biotypes. A sample of 109 B. tabaci female individuals obtained from 12 populations in Brazil were analyzed and compared to the A biotype from Arizona (USA) and B biotype from California (USA) and Paraguay. Trialeurodes vaporariorum and Aleurodicus cocois samples were also included. A total of 72 markers were generated by five RAPD primers and used in the analysis. All primers produced RAPD patterns that clearly distinguished the Bemisia biotypes and the two other whitefly species. Results also showed that populations of the B biotype have considerable genetic variability. An average Jaccard similarity of 0.73 was observed among the B biotype individuals analyzed. Cluster analysis demonstrated that, in general, Brazilian biotype B individuals are scattered independently in the localities where samples were collected. Nevertheless, some clusters were evident, joining individuals according to the host plants. AMOVA showed that most of the total genetic variation is found within populations (56.70%), but a significant portion of the variation is found between crops (22.73%). The present study showed that the B biotype is disseminated throughout the sampled areas, infesting several host plants and predominates over the A biotype.

A Presença de Plantas Invasoras Afeta a Infestação de Tomateiros por Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) em um Agro-ecossistema do Semi-árido?RESUMO -O manejo integrado de Bemisia tabaci (Gennadius) exige o conhecimento quantitativo dos mecanismos de persistência de populações em hospedeiros não comerciais. Este trabalho investigou a preferência para oviposição de B. tabaci entre tomate e quatro espécies de plantas invasoras, e a flutuação populacional do inseto em condições de campo no semi-árido nordestino. Semanalmente, foram determinados o nível (número de ovos e ninfas por folha) e a densidade de infestação (número de ovos e ninfas por área foliar) nas cinco espécies. Observamos diferentes graus de atratividade, formando um gradiente de preferência para oviposição entre as espécies, sendo Acanthospermum hispidum a espécie mais infestada e Amaranthus deflexus a menos infestada. Foi detectada ainda uma infestação significativamente mais elevada em parcelas contendo apenas tomateiros, sugerindo um efeito de diluição da infestação do tomateiro quando na presença de invasoras. Plantas invasoras podem representar importantes reservatórios para a mosca branca e/ou seus inimigos naturais, ou ainda fontes de inóculo viral para plantas cultivadas. Sua inserção em programas de manejo integrado de B. tabaci em tomateiro depende de como estes fatores podem ser manipulados para a redução dos danos causados pelo inseto.PALAVRAS-CHAVE: Mosca branca, planta daninha, preferência hospedeira, caatinga ABSTRACT -Integrated Bemisia tabaci (Gennadius) management requires quantitative knowledge on the mechanisms of persistence of pest populations in non-commercial host plants. We investigated B. tabaci oviposition preference among tomato and four weed species, and the insect population fluctuation under field conditions in the semi-arid region. At weekly intervals, we determined the level (number of eggs and nymphs per leaf) and the density of infestation (number of eggs and nymphs per leaf area) in all species. We observed an oviposition preference gradient among the species, with different degrees of attractiveness; Acanthospermum hispidum was the most infested species whereas Amaranthus deflexus was the least infested one. We also detected a significantly higher infestation on tomato when planted solely, which indicates a dilution effect on tomato infestation in the presence of weeds. Weeds can act as important reservoirs for whiteflies and/or their natural enemies or even as viral sources for cultivated plants. Their incorporation in integrated B. tabaci management programs depends on how those variables can be manoeuvred for the reduction of insect damage.

Although there are about 1200 species of whiteflies described, with less than 40 species being considered pests (Martin 1999), whiteflies have become a major problem for agriculture in tropical and subtropical areas of the world. Bemisia tabaci (Gennadius) and Trialeurodes vaporariorum (Westwood) cause the most severe losses (Byrne et al. 1990), and the occurrence of biotypes of B. tabaci has driven resources towards to the study of these insects (Martin et al. 2000).B. tabaci adaptability has resulted in extending its geographical range in subtropical and tropical agricultural systems and has facilitated its dispersal and establishment in temperate climate areas. Year-round crop production has favored whiteflies and has probably led to their survival throughout the year and further adaptation to new hosts (Brown 1994). This species is now globally distributed, being found on all continents except Antarctica (De Barro & Hart 2000).Whiteflies damage plants by feeding in large numbers, producing a sugary substance (honeydew) that accumulates on the leaves and fruits, on which sooty moulds develop, reducing photosynthesis and making the fruits inappropriate for commercialization (Hoelmer et al. 1994). The occurrence of whiteflies in Brazil was first registered at the end of the 19 th century (Göeldi 1886), but B. tabaci was only recorded in the 1930s (Bondar 1928). Occasional outbreaks of this species were described in the 1970s on bean, cotton and tomato (Costa et al. 1973). However, the strain or B biotype of B. tabaci was introduced into Brazil around 1991 through São Paulo State, possibly due to the commercialization of ornamentals (Lourenção & Nagai 1994). The B biotype rapidly spread in Brazil and now it occurs in 23 out of 27 states . The B biotype is found infesting a large number of crops and weeds, some of which have not been reported before .B. tabaci is a cosmopolitan and highly polyphagous species, T. vaporariorum is also cosmopolitan and polyphagous, and although it is considered one of the main greenhouse pests, in Brazil it is considered to be a secondary pest. However, T. vaporariorum has been recorded on 162 plant species, belonging to 40 families, in greenhouses in Brazil (Oliveira 1995).The aim of this paper was to present a preliminary list of B. tabaci and T. vaporariorum natural enemies collected in Brasília, DF, emphasizing the new occurrences. The study was not intended to provide quantitative data.Surveys of natural enemies were conducted from January 1999 Besides the colonies kept in greenhouses, an external area measuring 24 x 25 m, distant about 50 m from the greenhouses, and containing the same host plants mentioned above was also investigated periodically for natural enemy detection. In this area, the host plants were also disposed in rows. Weekly, 10 host plants leaves were randomly chosen and cut off. In the laboratory, the abaxial surfaces of the leaves were inspected and those infested by whitefly nymphs or containing predator larvae were kept in petri dishes and maintained in c...

Since 1999, a yellowing disease of melon ( Cucumis melo ) has been observed in the north-east region of Brazil. The symptoms, mainly seen on older leaves, include leaf mottling and yellowing, symptoms which are similar to those associated with criniviruses (Wisler et al ., 1998) or nutritional disorders. Deformation of leaves, stems and fruits was rarely observed, but the low sugar content had a high impact on fruit quality and value. The disease was transmitted from melon to melon (6/24) and to West Indian gherkin ( Cucumis anguria ) (6/24) by grafting and to melon by whitefly transmission ( Bemisia tabaci biotype B) (10/ 15). It was not mechanically transmitted. Examination of infected tissues by electron microscopy, using leaf dip and partially purified preparations, revealed filamentous particles 600 -700 nm in length and typical of a carlavirus. Typical carlavirus-like inclusion bodies were seen in melon leaves with yellowing symptoms, maintained in the glasshouse. In a dot-immunobinding assay using antibodies to Cowpea mild mottle virus , no cross-reaction was observed.As this is the only carlavirus reported to be whiteflytransmitted, this strongly suggests that this is a new carlavirus. Cloning and sequencing of the virus are underway and preliminary comparisons of sequences with those in the GenBank database showed homologies with some carlaviruses. As the possibility of a mixed infection with a crinivirus cannot be ruled out, we suggest the name Melon yellowing-associated virus (MYaV).