Analysis of free fatty acids in food substrates and in the dust and frass of stored-product pests: Potential for species discrimination?

https://doi.org/10.1016/j.jspr.2008.10.003Get rights and content

Abstract

The larger grain borer, Prostephanus truncatus, is a serious beetle pest that tunnels extensively to produce large quantities of dust and frass. The natural enemy Teretrius nigrescens is an important biological control beetle which is known to exploit at close-range solvent-extractable chemical cues in the dust and frass. The objective of the current study was to analyse quantitatively and qualitatively, the free fatty acid mixtures in different food-substrate materials both before and after insect attack by a range of stored-product pests in order to ascertain whether differences in these mixtures could explain the T. nigrescens selectivity to P. truncatus dust/frass over that of other species irrespective of food substrate. By TLC, GC and GC–MS we found triglyceride and five free fatty acids were the most abundant chemicals in dust/frass (palmitic acid (C16:0), stearic acid (C18:0), oleic acid (C18:1), linoleic acid (C18:2) and linolenic acid (C18:3)). In maize flour, Sitophilus species did not significantly change free fatty acid concentrations whereas with P. truncatus, Rhyzopertha dominica and Dinoderus minutus there were 4–6-fold increases, and, for Tribolium species there were over 20-fold increases. These differences provide interesting insights to tunnelling/feeding habits and are correlated with known feeding preferences within grain. Principal component analysis (PCA) demonstrated that free fatty acid ratios in dust/frass of different species are most linked to the food substrate and confer little discriminatory information that could be used to distinguish between the different species. Although increases in free fatty acid concentrations are good indicators of pest infestation and this may contribute behaviourally in an additive or synergistic way, we conclude that other chemical(s) are present and are key to T. nigrescens recognition of P. truncatus on different substrates.

Introduction

The larger grain borer, Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae), is the most serious pest of stored maize and dried cassava roots in Africa (Hodges et al., 1983, Dick, 1988). It tunnels extensively into these foods to produce large quantities of dust and frass (Fisher, 1950, Hodges, 1986, Rees et al., 1990, Stewart-Jones et al., 2004). Prostephanus truncatus is not native to Africa as it was first accidentally imported to the continent in the late 1970s, and in 1991 its Mesoamerican natural enemy Teretrius nigrescens (Lewis) (Coleoptera: Histeridae) was intentionally released into Africa for classical biological control (Dunstan and Magazini, 1981, Harnisch and Krall, 1984, Kalivogui and Mück, 1991, Richter et al., 1998).

Although long-range cues for prey-habitat location by T. nigrescens are well known (Rees et al., 1990, Böye et al., 1992, Helbig et al., 1992, Key et al., 1994, Scholz et al., 1998), what happens at closer range within the habitat is still being investigated (Stewart-Jones et al., 2004, Stewart-Jones et al., 2006, Stewart-Jones et al., 2007). It is now known that at close-range, adult and larval T. nigrescens are arrested by chemicals in the dust/frass of P. truncatus which are solvent extractable, and that these chemicals trigger intense searching behaviours (Stewart-Jones et al., 2006). Extracts of P. truncatus dust/frass collected from different tunnelling substrates are all similarly behaviourally active towards T. nigrescens, but extracts of dust/frass from other stored-product species are far less behaviourally active or even repellent (Stewart-Jones et al., 2007). We performed an initial chemical analysis of extracts of maize flour and P. truncatus dust/frass on maize by thin layer chromatography (TLC) and this indicated that P. truncatus infestation significantly increased the free fatty acid content in the dust (Stewart-Jones, 2002). Open column chromatographic separation of maize flour extract and extract of dust/frass from P. truncatus on maize followed by bioassays further showed some T. nigrescens behavioural activity towards the fraction containing free fatty acids (Stewart-Jones, 2002). This suggestion that free fatty acids might be important prey location cues is consistent with previous work in which P. truncatus dust/frass was either heated or stored for long periods and still retained biological activity, i.e. cues are not thermally labile and are of high boiling point (Stewart-Jones et al., 2004). We therefore hypothesised that the mixture of free fatty acids in the dust/frass of P. truncatus might provide cues for species specificity.

The objective of this study was to analyse, quantitatively and qualitatively, the free fatty acid content in different food-substrate materials both before and after insect attack. We analysed extracts from flours of maize, cassava and artificial substrates. We also measured changes caused by attack from seven stored-product beetle species by analysing extracts of their dust/frass after culturing on maize. The results for these analyses were then examined to investigate whether the blend of free fatty acids could be responsible for determining the selectivity of T. nigrescens to dust/frass from P. truncatus.

Section snippets

Food substrates and extracts of flours and dust/frass

Maize, cassava, artificial substrates, and insect details and methods for preparation of flours, dust/frass and respective extracts have been previously described in detail (Stewart-Jones et al., 2007). For clarity in this paper, flour was created using a mechanical mill (particles <500 μm) and dust/frass was sieved from insect cultures after 8 weeks (particles <500 μm). Maize was the usual food substrate but if alternative substrates were used a descriptive prefix is used. The prefix “pellet”

Analysis of extracts by thin layer chromatography (TLC)

When hexane extracts of maize flour and dust/frass from the seven different stored-product pest species reared on maize were analysed by TLC, two main spots were observed (Fig. 1).

The higher spot (R.f. 0.7) corresponded to the main component of maize oil. This was assumed to be a mixture of triglycerides, and pure trilinolenin (Sigma) run under the same TLC conditions did have the same R.f. (data not shown). As judged by the intensities and sizes of the spots on TLC, the quantities of

Discussion

We have performed a detailed quantitative and qualitative analysis of free fatty acids in the dust/frass of a range of stored-product pests. We show by TLC analysis that insect infestation significantly alters the amount of free fatty acids present in the dust/frass relative to the amount in maize flour that is produced mechanically. We have confirmed this by GC and GC–MS and have investigated whether there is enough discriminatory information for T. nigrescens to tell the species apart on the

Acknowledgements

This work was supported by funds granted to the University of Greenwich by the Higher Education Funding Council for England.

References (30)

  • R. Harnisch et al.

    Togo: further distribution of the larger grain borer in Africa

    FAO Plant Protection Bulletin

    (1984)
  • J. Helbig et al.

    Investigation on the distance of trapping activity of the synthetic pheromone “trunc-call (1 plus 2)” of Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae) on its predator Teretriosoma nigrescens Lewis (Coleoptera: Histeridae)

    Journal of Applied Entomology

    (1992)
  • R.J. Hodges et al.

    An outbreak of Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae) in East Africa

    Protection Ecology

    (1983)
  • M.D. Jellum

    Plant introductions of maize as a source of oil with unusual fatty acid compositions

    Journal of Agricultural and Food Chemistry

    (1970)
  • Cited by (4)

    View full text