Biological control of tomato leaf miner Tuta absoluta using entomopathogenic nematodes

Abstract

Increasing resistance to chemical insecticides in field populations of tomato leafminer, Tuta absoluta, has motivated research on alternative control measures. Biological control with entomopathogenic nematodes (EPNs) can be an alternative, particularly against larval instars. In foliar application, EPNs encounter many factors that affect their survival and efficacy adversely. This thesis has investigated: (1) the efficacy of some commercial EPNs against different stages of Tuta absoluta (larvae, pupae and adults) in Petri dish, leaf and soil bioassays, (2) factors affecting EPNs on tomato leaves and (3) impact of some organic and non-organic adjuvants on efficacy of EPNs at 75 ± 10 % and 45 ± 10 % RH. Results showed high adult and larval mortality of Tuta absoluta using Steinernema feltiae, Steinernema carpocapsae and Heterorhabditis bacteriophora in dish and leaf bioassays in optimum conditions (> 90 % RH). S. feltiae was the most virulent species, followed by S. carpocapsae then H. bacteriophora. Larval susceptibility increased throughout larval development. Pupae in contrast were less susceptible to nematode infection. In soil, S. feltiae and S. carpocapsae were significantly more virulent than H. bacteriophora against fourth larval instar when they drop to pupate in the soil and against adults when they are emerging from pupae. All factors studied (relative humidity, temperature, repeat application, method of application and time for EPNs to enter a leaf) affected the efficacy of EPNs to some extent, but relative humidity (RH) was the most important factor. EPNs’ efficacy and survival decreased as RH declined. The addition of Barricade® II and xanthan gum to aqueous suspensions of S. feltiae and S. carpocapsae resulted in increasing their efficacy and survival. What is more, they reduced the loss of nematodes into the soil in run-off by depositing them on the leaf and they assured even distribution of nematodes in the spray tank by preventing them from settling down. Moreover, Barricade® II (1.5 %) reduced the time required by S. feltiae and S. carpocapsae to enter a leaf. Barricade® II is the candidate adjuvant to be used with S. feltiae for foliar application against T. absoluta.