Integrated Management of Aphis craccivora in Cowpea Using Intercropping and Entomopathogenic Fungi under Field Conditions.

Cowpea aphid, Aphis craccivora, is a major cowpea pest. Cowpea-cereal intercrop alone does not effectively manage the pest. Use of pesticides in intercrop leads to health and environmental risks. Fungal-based biopesticides offer a better option because they are environment- and consumer-friendly. This study assessed the combined effect of Metarhizium anisopliae ICIPE 62 and cowpea-maize intercrop against A. craccivora under six treatments: (1) untreated cowpea monocrop, (2) untreated cowpea-maize intercrop, (3) cowpea monocrop + ICIPE 62, (4) cowpea-maize intercrop + ICIPE 62, (5) cowpea monocrop + Duduthrin insecticide, and (6) cowpea-maize intercrop + Duduthrin during three seasons (long rainy/cold and dry/short rainy). In the cold and dry season, cowpea-maize intercrop treated with ICIPE 62 recorded the lowest infestation/cowpea damage, whereas the leaf yield was comparable to cowpea monocrop treated with ICIPE 62. In the short rainy season, the cowpea-maize intercrop treated with ICIPE 62 recorded the lowest infestation/damage, whereas leaf yield was similar to cowpea-maize intercrop treated with ICIPE 62 in the cold and dry season. Duduthrin in monocrop and intercrop did not reduce aphid infestation/cowpea damage levels in all the seasons. Although the efficacy of M. anisopliae ICIPE 62-based biopesticide could be affected by seasons, it successfully controlled aphid population in cowpea-maize intercrop under field conditions without affecting aphid-associated natural enemies.

Endophytic Colonization of Onions Induces Resistance Against Viruliferous Thrips and Virus Replication.

In agricultural ecosystems, insect pests, pathogens, weather patterns, and reduced soil fertility pose major challenges to crop productivity and are responsible for significant yield losses worldwide. Iris yellow spot virus (IYSV) vectored by Thrips tabaci Lindeman, is a major hindrance to onion production in eastern Africa. Control measures often rely on insecticides with deleterious effects. Endophytes are one key alternative as they can play important roles in mediating induced systemic resistance. Hence, we examined the potential effect of endophytic fungus Hypocrea lixii (F3ST1) on feeding and replication of IYSV on endophyte-colonized (E+) and endophyte-free (E-) onion plants. For more precise assessment, replication was also tested using leaf disk bioassays and individual thrips. The number of feeding punctures was significantly lower in E+ as compared to E- plants. Disease level was significantly lower in E+ as compared to E- plants for four weeks post-exposure to thrips. IYSV replication was reduced by 2.5-fold in endophytic treatment on both whole plant and leaf disk assays. Thrips tabaci showed 2 times higher feeding activities on endophyte-free onion leaf disks as compared to the endophyte-inoculated leaf disks. Our results suggest potential utility of the endophytes to reduce feeding damage and virus infection on onion plants. Further studies should be conducted to elucidate the secondary metabolites involved in such endophyte-thrips-virus mediated interaction and determine whether the interactions extend for this and other onion varieties and viruses under field conditions.

Evaluation of the Entomopathogenic Fungi Metarhizium anisopliae, Beauveria bassiana and Isaria sp. for the Management of Aphis craccivora (Hemiptera: Aphididdae).

Cowpea, Vigna unguiculata ((L.) Walp; Fabales: Fabaceae), is an important indigenous vegetable and grain legume in the tropics where it represents a major diet component. Cowpea aphid, Aphis craccivora (Koch; Hemiptera: Aphididdae) is a major pest causing up to 100% yield losses. Aiming at establishing alternative approach to synthetic insecticides, we evaluated the pathogenicity of 23 fungal isolates including Metarhizium anisopliae ((Metschn.) Sorokin; Hypocreales: Clavicipitaceae), Beauveria bassiana ((Bals.) Vuill.; Hypocreales: Cordycipitaceae), and Isaria sp. (Hypocreales: Cordycipitaceae) against adult A. craccivora in the laboratory. Adult apterous aphids were sprayed with conidial suspensions titred at 1 × 108 conidia ml-1 for pathogenicity tests while 1 × 104, 1 × 105, 1 × 106, 1 × 107 and 1 × 108 conidia ml-1 were used in dose response bioassays. All the fungal isolates were found pathogenic to A. craccivora, causing mortality of between 34.5 and 90%. The lethal 50% mortality time (LT50) values varied between 3.3 and 6.3 d, with the best isolates being ICIPE 62, ICIPE 41 and ICIPE 644. The lethal concentration mortality (LC50) values were 2.3 × 106, 1.3 × 108 and 1.3 × 109 for ICIPE 62, ICIPE 41, and ICIPE 644, respectively. M. anisopliae isolate ICIPE 62 produced more conidia on aphid cadavers (4.5 × 107) than ICIPE 41 (2.7 × 107) and ICIPE 644 (2.1 × 107) 6 d post-treatment. Relative potency comparison showed that ICIPE 62 was more potent than the other two isolates. In the screenhouse, conidia of ICIPE 62 significantly reduced A. craccivora population compared to control but there was no significant difference between emulsifiable and aqueous formulations. Small-holder leafy vegetable producers could gain more profits using fungal-based biopesticides in Aphid-IPM strategies, leading to reduction of pre-harvest intervals after their application compared to synthetic insecticides.

Efficacy of Metarhizium anisopliae in controlling the two-spotted spider mite Tetranychus urticae on common bean in screenhouse and field experiments.

The efficacy of aqueous and emulsifiable formulations of the fungus Metarhizium anisopliae isolate ICIPE78 was evaluated on the population density of Tetranychus urticae infesting common bean plants under screenhouse and field conditions. Synthetic acaricide abamectin was included as a check. Bean plants were artificially infested with T. urticae and allowed to multiply. Three treatments were applied in the screenhouse and 1 treatment in field trials. Mite density was recorded 2 d before spraying and weekly postspraying. The number of pods per plant, number of seeds per pod, and the dry weight of seeds per plant were recorded only in the screenhouse trials. In both screenhouse and field trials, fungal formulations applied at the concentration of 10(8) conidia/mL and the acaricide reduced the population density of mites as compared to the controls. There were significant differences in T. urticae population densities between the treatments at the various post-spraying sampling dates. In the screenhouse, the mite densities were near zero from 3-week postspraying in the treated leaves. At 4-week postspraying, there were no more leaves in the untreated control (T1) and in the control water + Silwet-L77 (T2). Fungal formulations were as effective as abamectin in reducing mite densities in both screenhouse and field experiments. There were significant differences in the production parameters during the 2 screenhouse trials, with fungal and abamectin treatments generally having the highest yield. Results of this study underline the potential of the M. anisopliae isolate ICIPE78 as an alternative to acaricides for T. urticae management.

Optimizing modes of inoculation of Rhipicephalus ticks (Acari: Ixodidae) with a mitosporic entomopathogenic fungus in the laboratory.

The process of strain selection is an important step in the development of insect pathogens for biological control. Bioassays were conducted in the laboratory to evaluate the efficacy of different methods of inoculation using Rhipicephalus pulchellus Gerstäcker (Acari: Ixodidae) as a model. Initially, an oil-based formulation of Metarhizium anisopliae (Metsch.) Sorok. (Ascomycota: Hypocreales) titred at 10(9) conidia ml(-1) was applied to R. pulchellus adults using a Burgerjon spray tower or a microapplicator. Inoculation by microapplicator yielded poor results (25.0% tick mortality) compared to Burgerjon's spray tower (52.3% tick mortality), although the mean number of fungal conidia on R. pulchellus adults was lower (1.5 x 10(4) +/- 1.1 x 10(3) conidia ml(-1)) after spraying by Burgerjon's spray tower compared to 1 x 10(6) conidia ml(-1) obtained with the microapplicator. Thus, inoculation by Burgerjon's spray tower was selected for further investigations. Different modes of inoculation were tested and included direct spray of inoculum on the tick and substrate (SS), direct spray on the substrate and tick followed by transfer of the tick to clean uncontaminated Petri dish (SP) or indirect inoculation of ticks through substrate (SW). The LC(50) values following contamination of nymphs (LC(50) = 1.4 x 10(7) conidia ml(-1)) and adults (LC(50) = 6.7 x 10(7) conidia ml(-1)) in SS were significantly lower compared to SP; nymphs (LC(50) = 5.7 x 10(8) conidia ml(-1)) and adults (LC(50) = 5.3 x 10(9) conidia ml(-1)) and SW; nymphs (LC(50) = 5 x 10(8) conidia ml(-1)). Although the LC(50) value in SS was the lowest, it recorded the highest tick mortality among control ticks (24.2% at 2 weeks post-treatment) and (23.3% at 3 weeks post-treatment) in nymphs and adults respectively compared to SP (2.5 and 5.8%, respectively) and SW (0.0 and 0.0). Results show that among the modes of inoculation tested, SP was the most appropriate for inoculating R. pulchellus adults. SW and SP were identified as appropriate techniques for infecting the R. pulchellus nymphs with conidia formulated in oil.

Influence of temperature on virulence of fungal isolates of Metarhizium anisopliae and Beauveria bassiana to the two-spotted spider mite Tetranychus urticae.

Twenty-three isolates of Metarhizium anisopliae (Metschnikoff) Sokorin and three isolates of Beauveria bassiana (Balsamo) Vuillemin (Ascomycota: Hypocreales: Clavicipitaceae) were assessed for their virulence against the two-spotted spider mite, Tetranychus urticae Koch (Acari: Tetranychidae). Based on the screening results, nine isolates of M. anisopliae and two isolates of B. bassiana were tested for their virulence against young adult (1- to 2-day-old) female T. urticae at constant temperatures of 20, 25, 30 and 35 degrees C. At all temperatures tested, all the fungal isolates were pathogenic to T. urticae but mortality varied with isolates and temperatures. Fungal isolates were more virulent at 25, 30 and 35 degrees C than at 20 degrees C. The lethal time to 50% mortality (LT(50)) and lethal time to 90% mortality (LT(90)) values decreased with increased temperature. There were no significant differences in virulence between fungal isolates at 30 and 35 degrees C; however, significant differences were observed at 20 and 25 degrees C.