Teretrius nigrescens against larger grain borer Prostephanus truncatus in African maize stores: biological control at work?

Abstract

Summary 1.Following the accidental introduction of the bostrichid beetle Prostephanus truncatus into East and West Africa around 1980, a classical biological control campaign was launched in 1991; the histerid beetle Teretrius nigrescens was released as a biocontrol agent to prevent the destructive outbreak of the pest in small-farm maize stores. However, while the campaign has been ongoing, so has discussion in the scientific community about the merits of this campaign and its chances of success. 2. From published and unpublished data from experimental maize stores in Benin, West Africa, we derived statistical models describing the in-store insect population dynamics, and were thus able to point out significant biological interactions and to explain the observed lack of biological control. 3. We found that (i) T. nigrescens reduced significantly the population growth rate of both P. truncatus and the non-target pest, the weevil Sitophilus zeamais ; (ii) T. nigrescens displayed a positive numerical response to both prey species, P. truncatus and S. zeamais ; (iii) asymmetric competition existed between the two prey species, S. zeamais was negatively affected by P. truncatus but not vice versa; (iv) T. nigrescens and S. zeamais displayed negative intraspecific density-dependence whereas P. truncatus was resource-limited. 4. We conclude that classical biological control with T. nigrescens is not likely to become successful, mainly due to the predator's intraspecific density-dependence and its low population growth rate compared with its prey. We recommend that further research on P. truncatus integrated pest management takes into account the farmer as an active agent managing the store. 5. Synthesis and applications. When biocontrol does not result in satisfactory pest control, as in the case of P. truncatus , farmers should learn how to scout for the pest and take action when a need is detected. In areas where the pest is usually only a minor problem, the agricultural extension service should consider setting up a simple early warning system for their region. When attempts at classical biological control remain unsuccessful, as in the case of P. truncatus now for 10 years, policy-makers should prioritize training of extension service and farmers in integrated pest management techniques (e.g. needbased use of insecticides) as a necessary supplement to biocontrol.

Molecular markers reveal narrow genetic base and culturing-associated genetic drift inTeretrius nigrescensLewis populations released for the biological control of the larger grain borer in Africa

“…Colonies of T. nigrescens were founded by 200 individuals and the smallest releases were done with 5000 individuals (Giles et al ., 1996; Anonymous, 1999; P. Likhayo, KARI, Nairobi; I. Hoeschle-Zeledon, IITA, Ibadan, Nigeria, personal communication). Teretrius nigrescens has shown inconsistent establishment and control success in Africa, prompting some workers to conclude that the natural enemy is unsuitable for biological control (Holst & Meikle, 2003). Yet, a reduction in damage, LGB flight activity and grain loss was reported following the release of the predator (Hill et al ., 2003; Schneider et al ., 2004).…”

Biodiversity of Coleoptera

“…The Histeridae were used repeatedly as control agents against house flies (Musca domestica) (Kaufman et al 2000) and other flies with dung-inhabiting larvae (Davis 1994). Teretrius nigrescens, a histerid beetle, was released as a biocontrol agent against the bostrichid beetle Prostephanus truncatus to protect maize stores in equatorial Africa, although success was only moderate (Holst and Meikle 2003).…”

Efficacy of Beauveria bassiana against adults of Prostephanus truncatus (Horn), Sitophilus zeamais Motschulsky and Teretrius nigrescens Lewis in stored maize

“…In Tanzania, where this pest was first reported (Borgemeister et al 2003), on-farm storage losses of up to 34 and 70% over a 3 month and 4 months storage period have been reported for maize and dried cassava chips, respectively (Hodges et al 1983;Hodges et al 1985). It is currently the major storage pest of the two crops in Ghana (Obeng-Ofori 2008; Asante 2013) and Sub-Saharan Africa (Holst and Meikle 2003;Phiri and Otieno 2008). Some of the improved varieties of maize widely grown in Ghana including, 'Obatanpa' and 'Okomasa' with greater yields (Abdoulaye et al 2012) are equally susceptible to this pest (Golob et al 1999), therefore, suggesting an urgent need to search for an alternative, sustainable and environmentally friendly control measure.…”

“…Teretrius nigrescens is also known to feed on commodities with high starch content, for example wheat, sorghum and dried cassava (Pöschko 1993;Bonu-Ire 2015) hence may not be entirely beneficial (Anankware et al 2012). Holst and Meikle (2003) in a review of the impact of T. nigrescens as biocontrol agent against P. truncatus in West Africa concluded that, control using this predator might fail, as a result of the intra-specific density dependence and low population growth rate of T. nigrescens in comparison to P. truncatus. A more efficacious alternative to T. nigrescens is urgently needed.…”