Influence of hairiness of Gerbera jamesonii leaves on the searching efficiency of the parasitoid Encarsia formosa

To characterize the relationship between the leaf surface of Gerberajamesonii Hooker cultivars and the searching behavior of the parasitoid Encarsia formosa Gahan on a leaf, the trichome density and shape were described, and the walking behavior was quantified. Leaf hair density varied from 80 to more than 1000 trichomes/cm(2) and the hair shape varied from single erect trichomes to tapestries of entangled trichomes above the leaf surface. The walking activity, speed, and pattern of the parasitoid were tested on leaves with different hair density and shape. In spite of the large differences in leaf surface structure, the walking activity was similar and around 75% on most cultivars. The walking speed was between 0.2 and 0.3 mm/s on all cultivars and was not significantly different from the speed on tomato. On hairless sweet pepper leaves the walking speed is much higher (0.73 mm/s), so the hairs on Gerbera leaves do hamper parasitoid females and result in a strong reduction of the walking speed, but the variation in hair density and shape on Gerbera does not lead to differences in walking activity, pattern, and speed. It appears that the hairs of Gerbera are less of an obstacle for E. formosa than the stiff, large hairs occurring in a high density on cucumber, where the walking speed of the parasitoid is less than 0.2 mm/s. A rise of temperature of 5 degrees C gave a significantly higher walking speed (0.39 mm/s) of E. formosa females on all cultivars tested. The relative straightness of the walking track was high and the same on all cultivars. Consequences of the results of the searching behavior of E. formosa on G. jamesonii cultivars are discussed with respect to other host plants. As parasitoid walking speeds are the same on Gerbera and tomato, and whitefly reproduction is also similar on these two host plants, we conclude that biological control of whiteflies on Gerbera is a realistic option. This conclusion is confirmed by the strong increase of commercial biological control on Gerbera which occurred during the past 5 years. (C) 1997 Academic Press.