Ecology and population genetic structure of strains of Teretrius Nigrescens (Coleoptera: Histeridae), predator of Prostephanus truncatus (Coleoptera: Bostrichidae

Abstract

The larger grain borer (LGB) Prostephanus truncatus (Horn) is the most important pest of farm stored maize and cassava in Africa. This alien invasive species was introduced into the continent from Mesoamerica in the late 1970s and by 2008 had spread to at least 18 countries. In contrast to indigenous primary storage pests, LGB exists as on-farm and as wild populations, hence, sustainable control must target both environments. Biological control is especially attractive for wild populations to reduce early season grain store infestation, while cultural and chemical methods are useful to protect stored produce directly. Two populations of the predator Teretrius nigrescens Lewis were introduced into several African countries as a biocontrol agent. It has shown long-term success and cost effective control in warm-humid areas. Control has however not been successful in cool and hot-dry zones. The aim of this study was to investigate the possible underlying genetic and ecological explanations for these observations and the possibility of joint use of molecular markers and ecological parameters in the development of sustainable control strategies. A 28-month baseline monitoring and recovery activity was done in from 2004 in five regions in Kenya along an east-westerly transect. Monitoring and live sample collection was also done in the original outbreak area in eastern Kenya. There was greater LGB flight activity in western Kenya (high potential maize production area) than the low potential areas. Very few T. nigrescens were recovered, solely in the eastern regions. LGB flight activity followed a seasonal pattern mostly related to changes in the relative humidity at 12:00, rainfall and dew point temperature but with a 3 - 4 week lag. A linear predictive model based on these factors predicted 27 % of the observed flight activity. The survival and predation of five strains of T. nigrescens were compared at eight temperature levels between 15 °C and 36 °C at low and high humidity. All the strains of T. nigrescens exerted a significant reduction of LGB population build-up between 21 °C and 33 °C with generally better performance under humid conditions. There was no evidence of T. nigrescens development at 15 °C. At 18 °C, T. nigrescens oviposition and development was observed but the effect on LGB did not differ significantly from the control. The KARI population was the least effective in preventing grain damage at lower temperatures, but performed better than other strains above 30 °C at low humidity conditions. There was no control at 18 °C and 36 °C under both high and low humidity conditions. Since the extent of genetic differentiation in T. nigrescens was unclear from prior studies, several molecular marker techniques were progressively used. The RAPD-PCR did not reveal any genetic diversity between geographical populations. A 1000bp region of the mitochondrial mtCOI gene re vealed two distinct clades differing consistently at 26 segregating sites. The two clades can be identified by simple PCR-RFLP procedure using single or double sequential restriction with EcoR1, HincII, RsaI and DdeI digestion. However, the two lineages co-exist among the mid-altitude Central American populations. The internal transcribed spacer regions ITS1 and ITS2 with some neighbouring coding sequences of the ribosomal DNA were cloned and sequenced. The spacer regions were so variable in length and sequence between T. nigrescens and related Histeridae species that direct sequence alignment was not meaningful. Within T. nigrescens, there was intragenomic variability of the spacer regions mostly involving insertions and deletions of variable tandem repeat units predominantly within the ITS regions. The short flanking coding (18S, 5.8S and 21S) regions were conserved across populations and six other Histeridae species. There was no significant secondary structure variation of the ITS regions among populations of T. nigrescens. Twenty-four novel variable microsatellite markers were developed and tested on the Honduras populations. Alleles per locus ranged between two and twelve with observed heterozygosity between 0.048 and 0.646. Six loci deviated significantly from Hardy Weinberg Equilibrium and possibly had null alleles. The success of microsatellite amplification in outgroup species and variability of markers declined with an increase in the phylogenetic distance between the test species and T. nigrescens. Genotyping 432 individuals from 13 geographic populations revealed a comparatively higher genetic diversity in field populations. Partial isolation by distance and time was observed. Population bottlenecks were not detected, but recent expansion was evident in laboratory populations. Although five dominant genetic clusters were identified by Bayesian methods, meaningful hierarchical population structure was observed at between two and nine population groups (p < 0.01; 10,000 iterations). Biological control of the larger grain borer using T. nigrescens seems an important aspect of the sustainable integrated control approach of the pest. Ecological adaptations, appropriate release strategies and genetic diversity are all essential considerations in these efforts and could be responsible for the variable success already observed. There is some genetic differentiation between populations of T. nigrescens but, further studies would be necessary to ascertain the contribution of such diversity to its predatory performance. The effect of laboratory culturing in aggravating genetic drift should be accommodated to avoid loss of diversity during sampling, quarantine, rearing and release of the predator.