Effects of plant tissue on the acceptance of four greenhouse vegetable host by the greenhouse whitefly: An Electrical Penetration Graph (EPG) study

A combination of biological control and host-plant resistance is needed to control greenhouse whitefly, Trialeurodes vaporariorum (Westwood). The high level of susceptibility of 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, we use a method that allows an examination of the different tissue layers in the overall acceptance. The effects of plant tissue factors on whitefly probing profiles were monitored using the electrical penetration graph (EPG) method. The EPGs of 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 of first probes. An opposite probing profile was found on cucumber: longer probes, shorter xylem phases, fewer phloem phases but of longer duration, and longer first probes. The values of these parameters for gerbera and tomato were intermediate. Whiteflies encountered the greatest stimulation or the least resistance in the tissues of cucumber, and the least stimulation or the greatest resistance in the tissues of sweet pepper. Rejection of 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 of a host plant. Based on the performance of whitefly on these plants, which is also reflected in the values of the EPG parameters, the order of acceptance ranked from high to low is cucumber > tomato = gerbera > sweet pepper.