Entomologia Experimentalis et Applicata 102: 115–123, 2002. © 2002 Kluwer Academic Publishers. Printed in the Netherlands. 115 Stylet penetration behavior resulting in inoculation of a semipersistently transmitted closterovirus by the whitefly Bemisia argentifolii D. D. Johnson 1,2 , G. P. Walker 1,∗ & R. Creamer 3,4 Departments of Entomology 1 and Plant Pathology 3 University of California, Riverside, CA 92521, USA; Present address: 2 Gless Ranch, 18541 Van Buren Blvd., Riverside, CA92508, USA; 4 Department of Entomology, Plant Pathology, and Weed Science, Box 30003, MSC3BE, New Mexico State University, Las Cruces, NM88003, USA; ∗ Author for correspondence (Phone: 909-787-5808; Fax: 909-787-3086; E-mail: gregory.walker@ucr.edu) Accepted: January 8, 2002 Key words: Homoptera, Aleyrodidae, EPG, feeding behavior, virus transmission, lettuce chlorosis virus Abstract The electrical penetration graph (EPG) technique was used to determine what part of stylet penetration behavior by the whitefly vector, Bemisia argentifolii Bellows & Perring (Homoptera: Aleyrodidae), is lettuce chlorosis virus (LCV) inoculated in the host plant Malva parviflora L. LCV is a semipersistently-transmitted closterovirus. Since closteroviruses generally are found in the phloem of their plant hosts, this study tested the hypothesis that virus inoculation occurs during the phloem phase of stylet penetration behavior. Virus-exposed whiteflies were allowed to feed on uninfected host plants, and the whiteflies were divided into two experimental groups: group 1 attained phloem phase on the uninfected plants, and group 2 did not attain phloem phase. Two series of tests were conducted, one where whiteflies were manipulated so that the amount of time spent in non-phloem phase stylet penetration behaviors was similar between group 1 and group 2, and a second series of tests where whiteflies were manipulated so that the number of intracellular punctures made during stylet penetration was similar between group 1 and group 2. Both series of tests indicated that virus inoculation took place primarily during phloem phase. Considering only individual whiteflies shown to be capable of transmitting virus, 11 of 23 whiteflies (48%) in the phloem phase treatment successfully inoculated the virus whereas only one of 19 whiteflies (5%) in the non-phloem phase treatment successfully inoculated the virus (P = 0.00008). Introduction Semipersistently transmitted plant viruses are respon- sible for some of the most economically important dis- eases of cultivated crops, yet this group has received less attention in the scientific literature than nonpersis- tently and persistently transmitted plant viruses (Rac- cah et al.,1989). For a detailed description of transmis- sion characteristics for nonpersistently, persistently, and semipersistently transmitted viruses, see Nault (1997). The largest group of semipersistently trans- mitted plant viruses are the closteroviruses (Candresse & Martelli, 1995; Nault, 1997) which are transmit- ted by three families within the homopteran suborder Sternorrhyncha: Aphididae, Aleyrodidae, and Pseudo- coccidae (Nault, 1997). Each specific closterovirus is transmitted by only one or a limited number of vector species within the same insect family (Raccah et al., 1989). The vector behaviors responsible for transmission of plant viruses have been studied for numerous ho- mopteran vectors and the viruses they transmit. The earliest studies of stylet penetration behaviors that re- sult in nonpersistent transmission of potyviruses by aphids used transmission electron microscopy to ex- amine the stylet pathway in detail, and found that acquisition and inoculation depended on the aphid stylet tips puncturing plant epidermal cells (Lopez- Abella & Bradley, 1969; Lopez-Abella et al., 1988). More recent studies using the Electrical Penetration Graph (EPG) technique confirmed that transmission of nonpersistently transmitted potyviruses and cucu-