Selection, Optimization and Technical Application of Entomopathogenic Nematodes for the Biological Control of Major Insect Pests on Tomato

Abstract

Tomato is one of the most important crops worldwide. In Egypt, it occupies the largest cultivated area of vegetable crops. There are numerous insect pests attacking tomato crops in Egypt and they cause significant yield loss. The main tomato insect pests in Egypt are the Egyptian cotton leafworm Spodoptera littoralis (Boisduval), the black cutworm Agrotis ipsilon (Hufnagel), the cotton bollworm Helicoverpa armigera (Hubner), the Tomato leaf miner, Tuta absoluta (Meyrick), and the whitefly Bemisia tabaci (Gennadius). The mentioned insect species form a widespread complex which commonly attacks tomato crop in open fields and greenhouses, often more or less concurrently. Nowadays, the main control strategies are based on chemical insecticides with all known negative effects. Therefore, this thesis aimed to introduce a biological control system based on Entomopathogenic nematodes (EPNs) to be utilized against the “Egyptian Tomato insect Pest Complex” (ETPC) in order to have an environmentally sound alternative. The work started by screening 15 EPN isolates against the four lepidopteran pests (Chapter II). The objective of the performed screening was to find EPN isolates able to infect all ETPC effectively. The target insect species have many similarities such as their feeding behavior, the destructive stage, and the time of attack. Therefore, standard sand bioassays were performed at four doses against larvae of the target species. Based on the obtained results, the most virulent isolates were Steinernema carpocapsae BA2, S. feltiae Sf, S. abbasi abb, and S. carpocapsae J7. The second step was testing the efficacy of the selected isolates against the whitefly B. tabaci (Chapter III). The isolate S. feltiae Sf was the most efficient one against 2nd nymphal instars of the whitefly. The efficacy of the selected four EPN isolates was tested against T. absoluta in sand and tomato leaf bioassays (Chapter IV). T. absoluta was selected as the key target among ETPC because of its economic importance. Limited differences were recognised among the isolates when exposed directly in the sand bioassay. In leaf bioassays, all the tested isolates were capable of attacking T. absoluta larvae inside and outside the mines. The calculated LC50 values were 44 IJs/ml for S. carpocapsae BA2, 82 IJs/ml for S. abbasi abb, 103 IJs/ml for S. carpocapsae J7, and 112 IJs/ml for S. feltiae Sf. The next step was to find suitable EPN concentrations and adjuvants to increase their efficacy against T. absoluta larvae on tomato plants (Chapter V). The four isolates were applied at several concentrations and sprayed once or twice within 24 h on infested tomato plants. Applying the nematode suspensions twice resulted in significantly higher mortalities of T. absoluta larvae than sprayed once with double concentration. Except S. abbasi abb, EPN isolates were able to cause high larval mortality. When different formulations of S. carpocapsae BA2 were tested, the adjuvants Xanthan, Nemaperfect®, or Chitosan resulted in a significant increase in the larval mortality. These three adjuvants increased mortality from 70% (water) up to 88% (Xanthan). The adjuvant Nemaperfect® delayed nematode sedimentation in the suspension for about one hour. In greenhouse experiments, the four EPN isolates were applied twice within 24 h at 5000 IJs/ml in 0.3% Nemaperfect® as an adjuvant on tomato plants infested by T. absoluta larvae (Chapter VI). The highest larval mortality was achieved with the isolate S. carpocapsae BA2. There were no significant differences among S. carpocapsae BA2 (85.5%), S. feltiae Sf (80.5%), and S. carpocapsae J7 (76%), whereas S. abbasi abb resulted in significant lower mortality (18%). Based on the results of this extensive stepwise test program, it was possible to develop a biocontrol system against ETPC based on EPNs. Thus, the purpose of the thesis was achieved. The proposed system consists of S. carpocapsae BA2, S. feltiae Sf, or S. carpocapsae J7 in a concentration of 5000 IJs/ml with 0.3% Nemaperfect® or Xanthan. The application of the previous formulation twice within 24 h at dusk or at late afternoon could control T. absoluta and the other ETPC effectively. The next step should be the introduction and validation of this method in the practice of Egyptian tomato cultivation.