Effects of Neem and Pyrethrum on the Biology and Abundance of Liriomyza Huidobrensis( Diptera:Agromyzidae) and its Natural Enemy, Phaedrotoma Scabrieventris ( Hymenoptera:Braconidae)

Abstract

French bean crop (Phaseolus vulgaris L.) is a major export vegetable which is a source of revenue and income to the people of Kenya. Challenges facing French bean farmers are mainly pests. The pests includes the exotic leafminer of the genus Liriomyza (Diptera: Agromyzidae) which consists of Liriomyza huidobrensis, Blanchard, L. sativae Blanchard and L. trifolii Burgess. The most predominant of the three is L. huidobrensis, it is known to cause yield loss up to 70%. Control of L. huidobrensis is mainly by use of synthetic pesticides which is ineffective due to the quarantine status of the pest and its harmful effect on parasitoids. Botanical pesticides and use of parasitoids may offer a better alternative for the control of the Liriomyza flies.

An exotic parasitoid, Phaedrotoma scabriventris has recently been introduced in Kenya from Peru where it successively controlled Liriomyza flies. Due to the scarcity of information on the use of botanical pesticides and parasitoids towards the control of L. huidobrensis in Kenya, this study is undertaken to investigate the effect of neem and Pyrethrum on L. huidobrensis and the just released parasitoid P. scabriventris. The effect of neem and Pyrethrum on mortality, survival of L. huidobrensis larvae and emergence of P. scabriventris from parasitized pupae were evaluated both in the greenhouse and the laboratory at ICIPE.

Further field experiments were carried out at Sagana and Kabaru in Nyeri county of Kenya. Experiments on the effect of different concentration of Achook and Pyrethrum on feeding revealed a significant negative linear relationship between the number of feeding punctures and the concentration applied. The number of feeding punctures significantly reduced with increasing days before infestation in leaves treated with Achook and Pyrethrum while punctures in leaves treated with Neemroc increased significantly with days before infestation (OBI).

The mean number of eggs oviposited when leaves were treated with different concentrations of Achook, Neemroc and Pyrethrum did not have any significant differences. Similarly, the mean number of eggs on plants infested shortly, one and two days after treatment with Achook and Neemroc did not differ significantly but the eggs increased with number of days before infestation when the leaves were treated with Pyrethrum. Egg-adult survival increased as the concentration of Achook, Neemroc and Pyrethrum reduced. It also increased with increase in the number of OBI. Achook and pyrethrum concentrations were highly toxic to the I st larvae stages as % survival of the larvae reduced with increase in the concentration of Achook, Neemroc and Pyrethrum. Adult emergence from pupae of the I st, 2nd and 3rd instars increased with reduction in the concentration of the biopesticide. The % emergence of adults from pupae of the I st, 2nd and 3rd instars treated with Achook, Neemroc and Pyrethrum did not differ significantly.

There was a significant increase in the mortality of larvae in soil sprayed with Neemroc and Pyrethrum and infested shortly there after and one day later as compared to larvae mortality in soil sprayed and infested two days later. Effect of different concentrations of Achook, Neemroc and Pyrethrum on emergence of P. scabriventris and the LMF showed a significant reduction in emerged parasitoids as the concentration of the pesticide increased. The parasitoids were more tolerant to the chemicals than the LMF and their longevity was not affected. Field experiments at Kabaru and Sagana showed a significant higher % mortality of LMF when they were exposed to parasitoids and treated with four applications (A4) of Neemroc & Pyrethrum as well as in Farmers practice where Dirnethoate and Bulldock star was used. The total yield, percentage of unmarketable yield, leaf infestation, L. huidobrensis and P. scabriventris abundance registered no significance difference among all the application schemes applied.

This study therefore has clearly shown that low concentrations of Achook, Neemroc and Pyrethrum can be used to control L. huidobrensis especially in controlled environments without adverse effects on the parasitoids. However, field studies were only carried out for 8 and lO weeks in Kabaru and Sagana respectively. Therefore increased parasitoids release and recovery should be carried out frequently to monitor the establishment of the parasitoid and their spread to the surrounding regions.