Abstract:

Cowpea (Vigna unguiculata L. Walp) is one of the most important food and forage
legumes in the semi-arid tropics. The legume pod borer, Marttca vitrata (Fabricius)
(Lepidoptera: Crambidae) is considered as one of the most important insect pests
constraining cowpea production, causing yield losses of 20 - 80% across different parts
of the world. Use of chemical pesticides is the most widely known form of pest control on
cowpea. Exploitation of microbial pesticides has been gaining increased attention and
interest among those concerned with developing environmentally friendly and safe
approaches and tactics for pest management. This study aimed at evaluating the potential
of entomopathogenic fungi (EPF) for the management of M. vitrata on cowpea. Screening
of EPF was done to identify the most potent isolates against M. vitrata. Fourteen isolates
of Metarhizium anisopliae (Metschnikoff) Sorokin and six of Beauveria bassiana
(Balsamo) Vuillemin were screened against first instar larvae, from which the best two
isolates namely M. anisopliae ICIPE 18 and ICIPE 69 that caused highest mortality of
91% and 81%, respectively were selected for further studies. Assessment of biomass and
propagule production in four liquid media showed that isolate ICIPE 69 outperformed
ICIPE 18, while Jenkins-Prior and APUI ranked as best media. Thermotolerance studies
revealed that 25 °C and 30°C were optimal temperature for germination for ICIPE 18 and
ICIPE 69, respectively, while optimal temperature for growth were 30°C and 33°C for
ICIPE 69 and ICIPE 18, respectively. All isolates were virulent against M. vitrata at all
temperatures causing mortality of 56.0 — 91.6% across all temperatures (I5°C — 33°C).
Host plant effects were detected whereby insects raised on cowpea had the highest
mortality due to isolate ICIPE 18 (77.5%) while those reared on bean had the lowest
mortality (36.6%). On the other hand, virulence of ICIPE 69 was highest on beans
(95.7%), and lowest on Cajanus cajan. Cajanus cajan induced significantly fewer colony
forming units (CFU) for ICIPE 69. Fungal persistence was affected by plant, fungal
isolate, and time factors resulting in more than 90% reduction in CFU at 3 days post
spraying. Field evaluation results showed that Karate®, the commercial formulation of M.