The effect of Liriomyza trifolii (Burgess) (Dipt., Agromyzidae) on fruit production and growth of tomatoes, Lycopersicon esculentum (Mill) (Solanaceae)

The serpentine leafminer, Liriomyza trifolii (Burgess) (Agromyzidae) has, during the past 8 years, become an increasingly important pest on tomatoes, the second most important vegetable crop in South Africa. In some areas where weekly chemical applications are made on the basis of a threshold level of 0.25 mines per plant, it is feared that this pest has become resistant. In the present study, examination of the effect of various levels of infestation on growth and yield of tomatoes showed that neither growth nor yield were negatively affected by infestation levels of up to 1092 and 468 mines per plant in a glasshouse and field trial, respectively. A comparison of yield on control plants with 1–50, 51–100 and > 100 mines per plant (field trial) indicated that low L. trifolii infestations of 1–50 mines/plant in fact increased the yield by c . 60%. The phenology of L. trifolii feeding (before or during and after flowering) had no effect on yield and the effect of herbivory by L. trifolii was not obscured by any relationship between fruit production and growth of the tomato plants. These results were confirmed by a field trial and it is thus clear that even the threshold level being applied in the USA (four mines per three terminal leaflets per plant) is unrealistically low. Low correlations between number of mines per plant and percentage of pinnae or leaves infected indicated that assessing levels of infection by counting mines could not be replaced by the easier counting of pinnae or leaves infected.