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) h as, 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 o f a threshold level of 0.25 mines per plant, it is feared that this pe st has become resistant. In the present study, examination of the effe ct of various levels of infestation on growth and yield of tomatoes sh owed that neither growth nor yield were negatively affected by infesta tion levels of up to 1092 and 468 mines per plant in a glasshouse and field trial, respectively. A comparison of yield on control plants wit h 1-50, 51-100 and > 100 mines per plant (held trial) indicated that l ow L. trifolii infestations of 1-50 mines/plant in fact increased the yield by c. 60%. The phenology of L. trifolii feeding (before or durin g and after flowering) had no effect on yield and the effect of herbiv ory by L. trifolii was not obscured by any relationship between fruit production and growth of the tomato plants. These results were confirm ed by a held trial and it is thus clear that even the threshold level being applied in the USA (four mines per three terminal leaflets per p lant) 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.