An autodissemination strategy using entomopathogenic fungi and kairomonal attractants for managing thrips on grain legumes
Abstract
Grain legumes are among the key economical crops widely grown in western and eastern Africa
as important sources of food and animal fodder. However, the production of grain legumes in
Kenya is seriously affected by a complex of insect pests particularly thrips. Yield losses of 20 to
100% have often been reported in some areas. The bean flower thrips (BFT), Megalurothrips
sjostedti is considered to be the most important thrips pest of grain legumes. Chemical control is
still the main management strategy, with detrimental consequences on the environment, users
and consumers. Entomopathogenic fungi (EPF) are among the most promising alternatives to
chemical pesticides. Inundative sprays are the most common application techniques for EPF.
Although efficient and environmentally safe, the performance of entomopathogenic fungi is
affected by several environmental parameters such as UV light, temperature, drought and rain. In
order to improve the efficacy of EPF, an autodissemination system has been developed for the
management of thrips in greenhouses. In this system, thrips are attracted to an autoinoculator
where they are infected with an EPF before returning to the environment to disseminate the EPF
to conspecifics. It therefore provides promising prospects, but for effective control, the conidial
persistence and thrips attraction need to be optimized, while the EPF and the semiochemical
should be compatible. The objective of this study was therefore to optimize the
autodissemination system for thrips management on grain legumes in Kenya.
The semiochemical Lurem-TR, has been found to inhibit conidia of EPF when put together in an
autoinoculation device. The effect of spatial separation of Lurem-TR on the persistence of
conidia of EPF, Metarhizium brunneum and Metarhizium anisopliae was therefore evaluated to develop an autodissemination strategy for the management of M. sjostedti. Influence of spatial
separation of the semiochemical on thrips attraction and conidial acquisition by thrips from the
autoinoculation device was also investigated in the field. This study showed that conidia
persistence of both fungal species increased with distance of separation from Lurem-TR.
Attraction of thrips to the device also varied significantly according to distance between the
device and semiochemical. More thrips were attracted when Lurem-TR was placed in a container
below the device and at 10 cm distance from the device. Conidial acquisition by thrips was not
significantly different between spatial separation treatments of conidia and Lurem-TR.
Seven alternative thrips attractants, namely 4-anisaldehyde, ethyl benzoate, cis-jasmone, linalool,
methyl anthranilate, trans-caryophyllene and phenylethanol were also screened for their
compatibility with M. anisopliae ICIPE 69 in autodissemination devices and for their attraction
to M. sjostedti in the field. Methyl anthranilate (MA) was found to be the attractant most
compatible with M. anisopliae and its attractiveness to M. sjostedti was similar to that of Lurem-
TR.
The performance of the attractant, methyl anthranilate, was compared to the commercial
attractant Lurem-TR in autoinoculation devices treated with M. anisopliae under field conditions
for two seasons. Densities of M. sjostedti in plots with the two semiochemical-baited
autoinoculation devices were less than in the control plots during both experimental seasons.
Plots with MA-baited and Lurem-TR-baited devices had similar densities of M. sjostedti during
both seasons. However in the second season thrips densities in plots with the Lurem-TR-baited
devices did not differ significantly from the control plots. Conidial viability of M. anisopliae was significantly higher in semiochemical-free baited devices (control) than in semiochemical-baited
devices in both seasons. Conidial germination decreased over time in all the treatments but
remained above 45%, 12-15 days post-exposure. The average number of conidia acquired by a
single M. sjostedti ranged between 2.0 and 10.0 x 103 conidia in both semiochemical-baited
device treatments during both seasons. Significantly more conidia were acquired by single thrips
in MA-baited devices compared to Lurem-TR baited devices during the podding stage of the
crop during the second season. Significantly higher mortality of M. sjostedti was caused in field
plots by Lurem-TR baited and MA-baited autoinoculation devices compared to mortality of M.
sjostedti collected from the control plots in both seasons. Cowpea yield also differed
significantly between the treatment plots. The highest yield was recorded in plots where MAbaited
devices were placed. From this study, it could therefore be recommended that methyl
anthranilate be used in autoinoculation devices for the management of M. sjostedti on grain
legumes. The success achieved with MA in these trials resulted in the evaluation of this EPF for
possible use in a spot spray strategy.
The efficacy of spot spray and cover spray applications of M. anisopliae in combination with the
thrips attractant Lurem-TR was compared in field experiments for the management of M.
sjostedti on a cowpea crop in two seasons. Plants in the treatment plots where a spot spray
application of M. anisopliae was done five days after the placement of Lurem-TR recorded the
lowest densities of M. sjostedti. Fungal viability and thrips conidial acquisition did not differ
between the two application methods. Compared to the control treatment plots, both application
strategies resulted in yield increases of 34.1 and 46.2% with spot and cover spray treatments, respectively. The cost benefit analysis suggests that the spot spray application was more
profitable due to the reduction in labour and the quantity of inoculum used.