A bstract.A combination o f biological control and host-plant resistance is needed to control greenhouse whitefly, Trialeurodes vapo rariorum (Westwood). The high level o f susceptibility o f several host plants to whitefly, based on their performance on these plants, is well documented. These studies only provide information on the overall host-plant acceptance by whiteflies. Here, w e use a method that allows an examination o f the different tissue layers in the overall acceptance. The effects o f plant tissue factors on whitefly probing profiles were monitored using the electrical penetration graph (EPG) method. The EPGs o f whitefly originating from a culture on glasshouse cucumber, were recorded for 8 hours on sweet pepper, tomato, gerbera and cucumber plants produced in a glasshouse. On sweet pepper the graphs showed that whitefly made many short probes, had long xylem phases, short phloem phases, and the shortest duration o f first probes. An opposite probing profile was found on cucumber: longer probes, shorter xylem phases, fewer phloem phases but o f longer duration, and longer first probes. The values o f these parameters for gerbera and tomato were intermediate. Whiteflies encountered the greatest stimulation or the least resistance in the tissues o f cucumber, and the least stimulation or the greatest resistance in the tissues o f sweet pepper. Rejection o f host plants probably occurred before the phloem tissue was reached, as the probes prior to a whitefly leaving a host plant were so short that the stylets cannot have reached the phloem. But phloem factors also determine host-plant rejection, as phloem probing on sweet pepper -a poor host plant -was much shorter than on the other host plants. Resistance factors seem, therefore, to be located both in the epidermis/mesophyll and in the phloem. We hypothesize that the factors encountered by whitefly in the different tissue layers during probing contribute to the acceptance or rejection o f a host plant. Based on the performance o f whitefly on these plants, which is also reflected in the values of the EPG parameters, the order o f acceptance ranked from high to low is cucumber > tomato = gerbera > sweet pepper. INTRODUCTIONThe greenhouse whitefly, Trialeurodes vaporariorum (Westwood), is a serious pest of vegetables and ornamen tals growing in greenhouses and outside throughout the world (van Lenteren et al., 1992). This insect attacks more than 860 plant species from 469 genera and 121 families (Xu, 1994). Whitefly cause (a) direct damage by phloem sap ingestion, (b) indirect damage by transmitting plant viruses and other diseases (Cohen, 1990) and (c) decreased photosynthesis as a result of sooty moulds that develops on the honeydew excreted by the insect (Vet et al., 1980). One way of controlling whiteflies is through host-plant resistance . Complete host-plant resistance to insects has been found, and is employed against several insect pests, but is seldom per manent. Partial resistance against whitefly has been found in several plant species, ...

In order to analyze the variation in wood properties within and between trees of an underutilized tree species, we sampled six Oregon white oak (Quercus garryana Dougl.) trees from an 80-year old mixed stand of Q. garryana and Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) in the Coast Range of Western Oregon, USA. Fibre length, earlywood vessel diameter, tissue proportions, and specific gravity were measured on samples across the diameter at two heights. Trees had a slight lean (2-12°), so we sampled separately both radii of a diametric strip that ran from the lower to upper side of lean.

Different types of plant resistance against the greenhouse whitefly, Trialeurodes vaporariorum, were distinguished according to the tissue location of the resistance factors. The effects of resistance factors were compared by measuring the electrically monitored and visually observed probing and feeding behaviour of whiteflies on two resistant tomato genotypes (82207 and 82216), a susceptible tomato cultivar, and a non‐host plant, sweet pepper. On sweet pepper, whiteflies displayed very short first probes, very long pathway probing and spent much time on non‐feeding activities such as walking and standing still. Also, a high percentage of whiteflies rejected sweet pepper without ingesting substances from the phloem vessel. These data suggest a strong resistance that is based on the factors present in surface/epidermis and/or mesophyll layers of this plant. The behaviour of whiteflies on tomato 82207 was very different from that on sweet pepper. On tomato 82207 whiteflies apparently did not perceive resistance factors on the leaf surface and in the mesophyll. Resistance factors appeared to be present in the phloem tissue, because a higher number of phloem phases, longer phloem salivation periods and shorter phloem ingestion periods were observed when compared with the susceptible tomato cultivar. Tomato 82216 showed no clear resistance factor in the phloem. The importance of combined EPG and behavioural research for host‐plant resistance studies are discussed.

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