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In a previous study, we demonstrated that tomato resistance to Bemisia tabaci Gennadius (Hemiptera: Aleyrodidae), regulated by the Mi-1 gene, is dependent on the plant development, with statistically significant differences between carrying- and lacking-Mi-1 plants in 8-week old tomato plants but not in younger plants. Similarly, tomato resistance to the potato aphid is developmentally regulated meanwhile the Mi-1 resistance to root-knot nematodes is expressed in tomato plants as soon as 12 hours after seed germination. Now we present new results from another study comparing Motelle (with Mi-1) and Moneymaker (without Mi-1) cultivars to clarify whether the determinant factor for the expression of Mi-1 gene in tomato resistance against B. tabaci is actually plant age or plant size. In order to obtain plants with the same age but different size, a group of plants from each cultivar were grown at 24ºC and other plants were grown at a lower temperature (19ºC). When all plants were 60-day-old, they were moved to a greenhouse to be freely infested by uniform releasing of B. tabaci (Q-biotype) adults. The numbers of adults were counted daily on every plant until the new adults emerged. In the normal size plants, higher values from the parameters analyzed were obtained in Moneymaker than were in Motelle, but the differences decreased in the small plants. This indicates that not only plant age, but also plant size is an important factor to be considered in the tomato response to B. tabaci mediated by the Mi-1 gene.

The sweetpotato whitefly Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) is an important pest of horticultural crops worldwide. This insect causes damage directly through phloem feeding or indirectly by transmission of plant viruses such as Tomato Yellow Leaf Curl Diseases (TYLCDs). The tomato gene Mi-1.2 is responsible for resistance against the B and Q biotypes of B. tabaci, in addition to three species of root-knot nematodes (Meloidogyne spp.) and the potato aphid (Macrosiphum euphorbiae). It is known that tomato plants carrying Mi-1.2 are resistant to root-knot nematodes early in development: seedlings inoculated at 24 hours after germination are resistant to nematode infection. In contrast, the Mi-1.2-mediated resistance against potato aphid is developmentally regulated, with fully expanded leaves becoming resistant only when plants are 4 to 5 weeks of age. We have observed Mi-1.2-regulated resistance to B. tabaci in 8-week old tomato plants but not in other experiments with younger plants. However, a definitive study lacked to investigate whether Mi-1.2-mediated resistance against B. tabaci is developmentally regulated. Two sets of assays have been carried out in the present study, with tomato plants cv. Motelle (with Mi-1.2) and Moneymaker (lacking Mi-1.2 ) under free-choice and no-choice conditions, to determine if plant age or development could have any effect on whitefly resistance. In the first set of assays, 3-, 5- and 8-week old plants were compared. In the second set of assays, two groups of 8-week old plants were compared, which had been grown under different temperatures (24 or 19ºC) to obtain different plant developments.