Effects of Pesticide Application on the Population Dynamics of Liriomyza Huidobrensis (Diptera: Agromyzidae) and its Parasitoids on Peas (Pisum Sativum) in Central Kenya

Abstract

Snow Peas (Pisum sativum var saccharatum) and Sugar Snap (Pisum sativum var macrocarpon) are two of the most important legume-vegetables cultivated by small scale farmers in Kenya for both consumption and sale. The major production constraints are insect pests, diseases, nematode, insufficient rainfall and poor agronomic practices. The Liriomyza leaf mining flies (LMF) are among the most important pests causing a significant damage to these crops. For this reason, effective control of this pest is of utmost importance. Some of the methods that have been used in the control of L. huidobrensis include cultural practices such as crop rotation, as well as the use of synthetic pesticides. These pesticides have been used indiscriminately against these pests without considering their effects on the non-target pests. This study aimed at investigating the effects of pesticide application on the population dynamics of LMF and their indigenous natural enemies in central Kenya, one of the main production areas of peas in Kenya. On-farm field trials were set up at three different locations (Sagana, Kabaru and Naromoru) in Central Kenya. The experimental set up was a Randomized Complete Block Design. The treatments included: farmer practice where farmers were allowed to manage their plot following their routine and without restrictions on the pesticides use (FF), controlled pesticide where monitoring was done before pesticides could be applied (RP) and a control where no pesticides were used. In addition, laboratory experiments were designed to test the effect of commonly used pesticides (Dimethoate, Dynamec, Thunder, Cyclone, Bestox, Folicur, Milraz and Bulldock) on L. huidobrensis and its two parasitoids Diglyphus isaea and Phaedrotoma scabriventris. The mean numbers of leafminer were significantly higher in the farmers' field (60.9 and 79.6) than the control (30.6 and 18.8) and reduced pesticide application plots (25.4 and 29.8) in the first and second cropping seasons respectively. Local parasitoids were rare (0-25) in the field and despite the relative constant presence of the pest; the parasitoids were only present in a sporadic manner. However, the control plot recorded significantly higher numbers ofparasitoids (4.5 and 5.8) compared to the reduced pesticide (1.72 and 2.36) and Farmers' field (1.63 and 1.83) during the first and the second cropping periods respectively. Bestox, Cyclone and Dynamec were efficient in the laboratory against adult leafminers and their estimated LC50 (0.32, 0.5 and 0.24 mill respectively) were below the recommended doses (0.5, 0.75 and 0.5 mIll respectively). None of the pesticides was successful in controlling the larva of leafminers and the LCso for Bestox, Bulldock, Dimethoate, Dynamec and Thunder were 1.41, 1.64,.2.91, 1.16 and 2.94 mI/l respectively yet their recommended dosages were 0.5, 0.75, 1.5, 0.5 and 0.5ml/l respectively. Apart from Thunder which required 0.80 and 0.77 mI/l as the LC50 for D. isaea and P. scabriventris respectively against a recommended dosage of 0.5ml/l, all the other pesticides were harmful to the parasitoids at dosages below the recommended. For example, the for Bestox, Bulldock and Dynamec was 0.02, 0.01 and 0.02ml/l respectively. This study demonstrated that pesticide use in farmer fields had a role to play in decreased effectiveness of biological control by natural enemies. However, since the parasitoids already present in the field are few for effective control, there is need to increase the numbers through mass release. In addition, a study is needed on pesticides more friendly to the parasitoids such as botanicals.