Entomopathogenic Fungi: a Component of Leafminer Management

Abstract

The invasive leafminers Liriomyza sativae (Blanchard), Liriomyza trifolii (Burgess) and Liriomyza huidobrensis (Blanchard) (Diptera: Agromyzidae) are major pests of many vegetable and ornamental crops worldwide. In Kenya, production of horticultural crops is also severely constrained by infestation of Liriomyza leafmining flies (LMF), especially the invasive L. huidobrensis. Being quarantine pests, their presence in export produce can lead to rejections,resulting in loss of export markets and consequently loss of revenue to many smallholder families that are involved in export crop production. These constraints to trade represent the newest and potentially most challenging limitation to the future development of the horticultural sector in Kenya. Farmers increasingly use mixtures of chemical insecticides and spray more frequently in response to damage by key pests such as LMF. As a result, environmental contamination, health risks, pesticide residues and production costs are increasing. Increased use of pesticides also constrains the impact of the pest’s natural enemies and LMF have already developed resistance to several insecticides. The development of insecticide resistance has stimulated an increased interest in the search for non-chemical control measures such as the use of parasitoids, resistant plant varieties, entomopathogenic nematodes and entomopathogenic fungi as alternatives to chemicals. The current study is part of a larger research project on self-sustaining pest management strategies for Liriomyza species in Kenyan horticultural systems. The objective of this studywas to investigate the potential of entomopathogenic fungi Metarhizium anisopliae (Metchnikoff) Sorokin and Beauveria bassiana (Balsamo) Vuillem in (Hypocreales: Clavicipitaceae) for the control of L. sativae, L. trifolii and L.huidobrensis. The pathogenicity of 17 isolates of M. anisopliae and three isolates of B. bassiana to L. huidobrensis was evaluated in the laboratory. All the isolates were pathogenic to the leafminer causing mortality of 40 to 100% at five days post-exposure. The lethal time for 50% mortality (LT50) ranged from 2.6 to 5.4 days while the LT90 values varied between 3.2 and 9.1 days depending on the isolate. Ten isolates of M. anisopliae (ICIPE 315, 69, 78, 07, 60, 62, 84, 20, 387, 18) and three of B. bassiana (ICIPE 273, 603, 279) were found to be highly virulent. An autoinoculation device for field application of fungus was developed and tested in cage experiments using only one of the virulent isolates, M. anisopliae ICIPE 20. Mortality of up to 100% was observed in flies captured from fungus-treated cages held under laboratory conditions. This indicates that leafminer flies were attracted to the device and were able to pick up a lethal dose of inoculum (4.1 x 105 to 4.0 x 106 conidia per fly), resulting in high adult mortality. One day after the inoculation, adults picked-up an average of 4.1 ± 0.7 x 105 conidia and 39.6 ± 4.0 x 105 conidia five days post-inoculation. Depending on the sampling date, the LT50 varied between 1.8 and 3.4 days. The effect of fungal infection by M. anisopliae ICIPE 20 on feeding and oviposition of adult L. huidobrensis was examined on three host plants, i.e. faba bean, Vicia faba L., snow pea, Pisum sativum L. (cv. Oregon II) and French bean, Phaseolus vulgaris L. (cv. Samantha) (Fabales: Fabaceae), in the laboratory. Infection by M. anisopliae significantly reduced feeding and oviposition by L. huidobrensis. However, reductions in punctures and eggs generally occurred after 72 h postinoculation.The host plant did not have any effect on the feeding but had an influence on the egg laying, with faba bean harboring a greater number of eggs in both the control and the M. anisopliae treatments. Insects reared on faba bean were less susceptible to fungal infection than those reared on French bean and snow pea. The effect of constant temperatures on the virulence of five isolates of M. anisopliae against the three species of leafminers, L. huidobrensis, L. sativae and L. trifolii was studied in the laboratory. Insect mortality varied with temperature, fungal isolate and leafminer species. Results showed that fungal isolates were more virulent at 25C and 28C than at 15C and 20C and that, lethal time to 50% mortality (LT50) values decreased with increasing temperature. In another set of experiments, the effect of two host plants, French bean and faba bean on the susceptibility of the three leafminer species to one of the virulent M. anisopliae isolates (ICIPE 20) was also investigated. Leafminer mortality was affected by both the concentrations used (1 x 105, 106, 107 conidia ml-1) and the host plant. Insects reared on faba bean plants seemed to take longer to succumb to infection. Liriomyza huidobrensis reared on faba bean required higher conidial concentrations to kill compared to those reared on French bean. However, for both L. sativae and L. trifolii, host plant had no effect on concentration. The effect of the entomopathogenic fungus M. anisopliae isolate, ICIPE 20 on the leafminer parasitoid Diglyphus isaea (Walker) (Hymenoptera: Eulophidae) was investigated in the laboratory. Results showed that M. anisopliae was pathogenic to D. isaea adults causing up to 76% mortality at six days after inoculation and affected the emergence of the parasitoids. Results of this study demonstrate the potential of entomopathogenic fungi M. anisopliae and B. bassiana for the management of L. huidobrensis, L. trifolii and L. sativae.