A System of Caging Thrips palmi for Laboratory Bioassay of Pathogens

Abstract

Thrips palmi Karny (Thysanoptera: Thripidae) is an Asian species considered native to the Malaysian-Indonesian region. It is now found throughout Asia, Africa, Central and South America and the Caribbean. T. palmi has been present in the Caribbean since 1985 (Anonymous 1989) and is a serious threat to vegetable production in the region. It attacks over fifty plant species including eggplant, sweet peppers, cucumbers, melons, pumpkin, squash and beans as well as other commercially important crops (Wang & Chu 1986). Reports from Puerto Rico, Guadeloupe and Trinidad and Tobago indicate that T. palmi caused losses ranging from 50-90% in vegetable plantings (Franqui et al. 1991, Etienne and van Watermuelen 1991, Jones 1990). Chemical control of T. palmi has not been successful due to resistance to a range of insecticides (Guyot 1988, Jones 1990). Consequently, we are currently evaluating pathogens for the control of T. palmi. So far, a new species of Hirsutella Patouillard has been discovered attacking T. palmi in Trinidad (Hall, 1992) and other fungi such as Verticillium lecanii (Zimm.) Viegas and Beauveria bassiana (Bals.) Vuill. cause disease in related Thrips species (Binns et al. 1982, Gillespie. 1986). In order to simply, reliably and accurately bioassay the abilities of such pathogens to infect T. palmi, a new caging system has been developed and is described in this note. In order that a caging system for pathogen bioassay be of value, the following criteria must be met: (i) the insects must be individually contained to eliminate the risk of cross-contamination; (ii) the insect should be able to move freely within the chamber; (iii) the system should permit clear observation of the insects at all times; (iv) there should be sufficient air exchange to supply the needs of the insect and leaf tissue and to prevent condensation (in which small insects such as thrips may drown) on the walls of the chamber or on the leaf discs; (v) the substrate, normally leaf tissue, must be maintained in a palatable form for the duration of the bioassay; (vi) there should be minimal handling of the insect; (vii) the control mortality must be within acceptable limits (below 10%); (viii) the cages must be escape-proof and; (ix) the system must be reasonably simple to set up so that large numbers of assays may be performed without undue difficulty. Meeting these criteria is especially important when working with a small, highly mobile insect such as T. palmi. There is a variety of caging systems for thrips. Most of these have been developed to assay insecticides, to study life-cycles or rear thrips. These range from group rearing on whole leaves in mesh and plastic clip or sandwich cages to individual rearing in glass vials such as the Plaster of Paris block cage (Lewis 1973). The latter cage contains pieces of leaf or grass spikelets kept in glass tubes with one end embedded vertically in plaster and the other plugged with cotton wool wrapped in gauze to prevent the thrips from becoming entangled in the fibres. This is an excellent technique, as are those described by Morse et al. (1986) and Teulon (1992) but all these methods have some disadvantages such as becoming unwieldy when dealing with large numbers of