Plant extracts have been tested mainly in the form of essential oils against stored grain pests and, in some cases, the research has extended as far as including the main components of these oils.
Regarding mites, studies have addressed the effects on pests, as well as predatory species. There are three substances that have shown the best results when tested in the lowest of concentrations, even lower than benzyl-benzoate (an already industrially available product). These substances are components of the clove bud (E. caryophyllata) essential oil, methyleugenol, forget-me-not (M. arvensis) essential oil and benzaldehyde of P. persica. Furthermore, cinnamon (naturally containing benzyl-benzoate) has been proven to be effective against T. putrescentiae, T. molitor, S. oryzae, R. dominica, P. interpunctella and E. kuhniella as a contact toxicant to larvae or adults, as a repellant or as a fumigant. However, detailed knowledge of the lethal or non-lethal effects of botanical pesticides on beneficial insects is important for integrated pest management (IPM).
Regarding the impact of pesticides on Coleoptera and Lepidoptera, many plant species have been studied for their pesticidal properties. C. copticum was found to be active against T. confusum, S. granarius, P. interpunctella and E. kuehniella, whereas basil was effective as a fumigant for S. zeamais, O. surinamensis and E. kuehniella. Most interestingly various Citrus species were effective as fumigants, contact toxicants and as repellants, for both Coleopterans (T. castaneum, S. oryzae and C. ferrugineus) and Lepidoptera (P. interpunctella, S. cerealella and E. kuehniella). In addition, in regard to Lepidoptera, many substances have been evaluated for sublethal effects (C. cincta lowered the percentage of adult emergence, and A. sativum reduced adult longevity and inhibited oviposition), whereas for Coleoptera only a few studies of sublethal effects are available.
T. minuta and T. patula extracts were very effective fumigants against T. castaneum Herbst, whereas Eucalyptus spp. extracts were very effective fumigants against the adults of R. dominica, T. castaneum and O. surinamensis, but were also very effective against Lepidoptera (E. kuehniella, E. cautella, C. cephalonica and P. interpunctella) as fumigants or repellents. C. reticulata and C. sinensis essential oils were among the few substances found to be effective against Coleoptera (T. castaneum) larvae, as was the garlic essential oil, causing necrosis to the larvae, pupae and adults of T. molitor L. The fumigant properties of C. citratus and Z. officinale were the most effective fumigants against the rice weevil. Furthermore, A. judaica and C. viminals had two of the lowest LC50 values observed in this entire review. L. salicifolia, L. nobilis and E. floribundi were significantly effective repellents, whereas O. acutidens and C. reticulata were effective fumigants against L. serricorne and C. ferrugineus.
Finally, regarding Lepidoptera, the essential oil of C. anisata was found to be completely toxic to the larval stages, and also inhibited the emergence of adults. M. piperita and S. officinalis incorporated into polylactic acid solution showed significant contact toxicity on P. interpunctella, whereas M. piperita showed higher toxicity than S. officinalis. The essential oils exhibiting contact toxicity showed higher toxic effects when they were loaded on polymeric nanoparticles, as in the case of palmarosa (C. martinii), geranium (G. maculatum), and peppermint (M. piperita) against P. interpunctella. Eucalyptus spp. was the most tested substance in Lepidoptera, achieving 100% mortality, and it was effective as a fumigant and exhibited contact toxicity. Additionally, purslane essential oil was very effective against E. cautella in terms of fumigant toxicity, oviposition deterrence and persistence against larvae and adults. M. tennuifolia achieved 100% E. cautella moth mortality and reduced the hatchability of the eggs, whereas adult emergence was prevented, making it a very useful tool against E. cautella; as was birch essential oil, which displayed high fumigant toxicity on the eggs of E. kuehniella. Furthermore, A. graveolens oil was found to be the most active against E. kuehniella larvae, the life stage of the Lepidoptera species that causes the most damage. Finally, the essential oil of O. vulgaris was found to affect pupal development and disrupt the reproduction of exuviated adults by extending the preoviposition period and reducing the period of egg-laying and fecundity.
In general, there is a shortage of tools for combatting stored-product pests, as an alternative to the use of synthetics. With this review, we refer to the botanical substances reported in the literature of recent years to exhibit pesticidal properties of potential utility in the management of stored products (Table 1).
Table 1. Plant species reported to exhibit significant activity against stored-product pests in recent years.