Within‐plant distribution and infestation pattern of the B‐ and Q‐biotypes of the whitefly, Bemisia tabaci, on tomato and pepper

Two biotypes (B and Q) of the sweetpotato whitefly, Bemisia tabaci (Gennadius) (Homoptera: Aleyrodidae), are present in Spain (Guirao et al., 1997). These whiteflies cause serious damage by feeding on the underside of leaves and by transmitting devastating plant viruses, such as tomato yellow leaf curl viruses (Carnero et al., 1990; Brown & Bird, 1996; Moriones et al., 1993; Bedford et al., 1994; Blua & Toscano, 1994; Markham et al., 1996; Jiang et al., 1999). Although both biotypes are major pests of horticultural crops in many of the tomato and pepper-growing areas, recent studies on some hosts have shown that the Q-biotype infests a higher percentage of plants and develops faster than the B-biotype (Muniz, 2000; Nombela et al., 2001). Although growers use insecticides to control whiteflies, no monitoring technique has been specifically developed for the Q-biotype. Reliable sampling methods are critical to the development of monitoring programs for pest management application (Naranjo & Flint, 1995; Naranjo, 1996; Ellsworth & Martinez-Carrillo, 2001). Location of whiteflies on the different plant strata and accurate estimation of the infestation capacity (percentage of plants infested by a certain number of insects) of B. tabaci are important aspects in the study of its biology and ecology. In the last ten years, several studies have determined the spatial within-plant distribution of immature stages of B. tabaci in many important crops such as cotton (Butler & Vir, 1990; Rao et al., 1991; Naranjo & Flint, 1994, 1995), peanut (Lynch & Simmons, 1993), melon (Tonhasca et al., 1994), tomato (Carnero & Gonzalez-Andujar, 1994; Shuster, 1998), and alfalfa (Yee et al., 1997). The immature stage may be the stage that is most correlated to plant damage, but immatures are difficult and timeconsumming to count. Consequently, it is likely that the adult stage will continue to be the center of focus for pest management application (Naranjo, 1996). To our knowledge, no reports exist on the within-plant distribution of B. tabaci adults in pepper and tomato. Another main aspect of the spatial distribution (whose knowledge could help growers to make correct decissions on whiteflies control) is between-plant or within-field distribution which is typically described with a theoretical distribution such as the Poisson or negative binomial, or with models as Taylor’s power law (Taylor, 1961) or Iwao’s patchiness regression (Iwao, 1968) which relate the mean and variance over a range of population densities (Naranjo, 1996). In addition, there has been limited effort to describe the inter-relationships between different biotypes of B. tabaci and their host plants (Muniz, 2000; Muniz & Nombela, 1997a,b; Nombela et al., 2000, 2001). Our objectives were to obtain, under greenhouse conditions, the within-plant distribution pattern of both B-and Q-biotype adults of B. tabaci for tomato plants, and to investigate the relationship between the proportion of infested plants and the number of adults present on pepper or tomato plants by a descriptive model under specific greenhouse conditions.