Biological control of soil-dwelling insect pests in cocoa agroforests using CO2-emitting capsules coformulated with entomopathogenic fungi

Abstract

Termites have recently gained importance as major pests in cocoa agroforests (AF) because of a loss in overall biodiversity at the transition from shaded agroforestry system to intensively managed unshaded monocultures (full sun) systems. Termite control relied almost exclusively on persistent organochlorine insecticides which are currently under restrictive use due to increasing concern over damage to human health and the environment. Entomopathogenic fungi (EPF) are considered as promising biocontrol agents in inundative augmentative biocontrol strategies against termite pests. However, there are limitations in their application as they do not achieve high control efficacies in the field when applied as conidial suspensions due to repellency, host avoidance, and defense mechanisms against virulent EPF. Subterranean termites use CO2 to locate plant roots, thus making the use of EPF a promising biocontrol strategy against termites when combined with CO2 in a strategy known as attract and kill (A&K) or Attract and Infect (A&I). This study was therefore undertaken to explore the potential efficacy of encapsulated CO2-emitting material co-formulated with a virulent EPF (Metarhizium brunneum (Metschnikoff) Sorokin) for biological control of termites in cocoa agroforests. The first objective of this study focused on a review of soil-dwelling insect pests of tree crops in sub-Saharan Africa where termites were identified as the major soil-dwelling insect pests affecting tree crops and have recently gained importance as major pests in cocoa agroforests. The study further compared termite assemblages under five cocoa agroforestry shade types in Cameroon to assess the impact of shade on termite taxonomic and functional group diversity and to identify the termite species causing damage to cocoa. Sixty-nine termite species in 33 genera, 5 subfamilies under 2 families were sampled. Termite species richness decreased significantly from the shaded cocoa AF (92.54% shade cover), dominated with soil feeders or non-pest species to the full sun AF systems (22.5% shade cover),dominated with pest species. Functional group composition was strongly correlated with variation in shade level, with functional group III and IV representing the most abundant in the shaded systems and rare in the low shade and full sun systems. The shaded AF systems maintained all the termite species found in the full sun system and causing damage to cocoa trees. The shaded systems also harboured a diversity of non-pest species, suggesting that the establishment of shade in cocoa AF conserves important part of functional biodiversity. Screening to select virulent EPF fungi to co-formulate with CO2 generating materials for control of subterranean termite pests in cocoa agroforests was conducted. The results showed that Metarhizium isolates were more virulent with lower LT50 values than Beauveria isolates, M. brunneum Cb15-III being the most virulent (LT50 = 1.5 days). The study further investigated whether calcium alginate beads containing baker’s yeast (Saccharomyces cerevisiae Meyen ex Hansen) as an encapsulated CO2 source (CO2-emitting capsules) could outcompete CO2 gradients established by other CO2 generating materials and other attract components to attract subterranean termites (Microtermes spp.). The capsules co-formulated with the highly virulent EPF M. brunneum: Cb15-III (CECEPF) were further assessed for their ability to establish CO2 gradients in the soil that can outcompete CO2 produced by cocoa seedlings root respiration to attract and consequently kill termites. In addition, infection of the worker termites by the fungal spores growing from the CECEPF as well as their horizontal transmission was investigated through the autodissemination approach. Significantly more termites were attracted to CEC compared to other attract components. No significant difference was observed in the number of termites attracted by CECEPF and cocoa seedlings. The capsules were further tested under semi field and field conditions for their attractiveness to termites. Under the semi field condition, no significant differences were observed in the number of termites collected around cocoa seedlings in control and treatment plots when CEC or CECEPF were introduced into treatment boxes. Similarly, for the field trials, no significant difference was observed in the number of attractive stations found with termites in the control and treatment plots during the study period, as well as in the mortality of seedlings. The “attract and kill” strategy therefore offers a high potential to promote biological termite control in cocoa agroforests as an alternative to insecticides.