Attractiveness of oilseed rape cultivars to Brassicogethes aeneus and Ceutorhynchus obstrictus as a potential control strategy

The abundances of two pests, pollen beetle (Brassicogethes aeneus (Fabricius, 1775)) and cabbage seed weevil (Ceutorhynchus obstrictus (Marsham, 1802)), were measured before flowering and in the full bloom of oilseed rape cultivars with different phenologies (two yellow-flowering: DK Exssence (the earliest), DK Sensei (the latest) and one white-flowering in time between yellow-flowering cultivars), and the differences in their abundance in the selected cultivars were determined in plot experiments during 2015–2018. No significant differences in pollen beetle and cabbage seed weevil occurrence were observed between the cultivars in the pre-flowering period, but during flowering, the two yellow-flowering cultivars were more attractive than the white-flowering cultivar for both pests. In the white-flowering cultivar, 57% and 69% reductions in the pollen beetle and cabbage seed weevil populations, respectively, were found relative to the two with yellow flowers. Thus, the use of white-flowering cultivar (less attractive, later flowering) as the main crop and the yellow-flowering cultivar (more attractive, earlier flowering) at field edges, with the width of the one-track line, could serve as a strategy to manage oilseed rape pests during flowering. This control strategy which combines more and less attractive oilseed rape cultivars may contribute to a reduction in the use of pesticides and their negative impact on the environment.

The effect of insecticide application by dropleg sprayers on pollen beetle parasitism in oilseed rape

Dropleg sprayers apply pesticides below the flower horizon of oilseed rape plants and thus reduce unwanted side effects on pollinating insects. Whether this technique benefits parasitoids of seed and pollen feeding insect pests has not been studied earlier. To answer this question, we first assessed the vertical distribution of pests and parasitoids using a portable aspirator. In addition, parasitism rates of pollen beetle, Brassicogethes aeneus Fabricius (Coleoptera: Nitidulidae), by the larval parasitoid Tersilochus heterocerus Thomson (Hymenoptera: Ichneumonidae) were compared in conventional and dropleg sprayed fields over four years (2016–2019), using the neonicotinoids thiacloprid and acetamiprid. Our results show that seed and pollen feeders were mainly found in the flowering canopy, while the predominant location of parasitoids was species-specific. Among pollen beetle parasitoids, Phradis interstitialis Thomson (Hymenoptera: Ichneumonidae) was more abundant below flowering canopy (63% of total catch), whereas T. heterocerus was mainly caught in the flowering canopy (84% of total catch). In the spraying experiments, average parasitism rates of pollen beetles by T. heterocerus ranged between 55 and 82% in the untreated controls. In the dropleg spray treatments, parasitism rates did not differ significantly from control levels, with the exception of thiacloprid application in 2019. In contrast, conventional spray applications resulted in a reduction of parasitism rates by up to 37% compared to the control for at least one of the insecticides in three out of four years. The impact of conventional application differed between years, which may be explained by the temporal coincidence between spray application and the immigration of parasitoids into the crop. We conclude that dropleg spraying exerts lower non-target effects on the main biological control agent of pollen beetle.

RNAi efficacy is enhanced by chronic dsRNA feeding in pollen beetle

Double-stranded RNAs (dsRNAs) represent a promising class of biosafe insecticidal compounds. We examined the ability to induce RNA interference (RNAi) in the pollen beetle Brassicogethes aeneus via anther feeding, and compared short-term (3 d) to chronic (17 d) feeding of various concentrations of dsRNA targeting αCOP (dsαCOP). In short-term dsαCOP feeding, only the highest concentration resulted in significant reductions in B. aeneus survival; whereas in chronic dsαCOP feeding, all three concentrations resulted in significant mortality. Chronic dsαCOP feeding also resulted in significantly greater mortality compared to short-term feeding of equivalent dsαCOP concentrations. Our results have implications for the economics and development of dsRNA spray approaches for managing crop pests, in that multiple lower-concentration dsRNA spray treatments across crop growth stages may result in greater pest management efficacy, compared to single treatments using higher dsRNA concentrations. Furthermore, our results highlight the need for research into the development of RNAi cultivars for oilseed rape protection, given the enhanced RNAi efficacy resulting from chronic, compared to short-term, dsRNA feeding in B. aeneus.

Targeting a coatomer protein complex-I gene via RNA interference results in effective lethality in the pollen beetle Brassicogethes aeneus

The pollen beetle Brassicogethes aeneus is a serious pest of oilseed rape (Brassica napus) in Europe. Management of this pest has grown difficult due to B. aeneus’s development of resistance to pyrethroid insecticides, as well as the pressure to establish control strategies that minimise the impact on nontarget organisms. RNA interference represents a nucleotide sequence-based, and thus potentially species-specific, approach to agricultural pest control. The present study examined the efficacy of targeting the coatomer gene coatomer subunit alpha (αCOP), via both microinjection and dietary exposure to exogenous complementary dsRNA, on αCOP-silencing and subsequent mortality in B. aeneus. Beetles injected with dsRNA targeting αCOP (at 0.14 µg/mg) showed 88% and 100% mortality at 6 and 10 days post-injection, respectively; where by the same time after dietary exposure, 43%–89% mortality was observed in the 3 µg dsRNA/µL treatment, though the effect was concentration-dependent. Thus, the effect was significant for both delivery routes. In working towards RNA-based management of B. aeneus, future studies should include αCOP as a target of interest.

First Evidence of Bud Feeding-Induced RNAi in a Crop Pest via Exogenous Application of dsRNA

Spray-induced gene silencing (SIGS) is a potential strategy for agricultural pest management, whereby nucleotide sequence-specific double-stranded RNA (dsRNA) can be sprayed onto a crop; the desired effect being a consumption of dsRNA by the target pest, and subsequent gene silencing-induced mortality. Nucleotide sequence-specificity is the basis for dsRNA’s perceived biosafety. A biosafe approach to pollen beetle (Brassicogethes aeneus) management in oilseed rape (Brassica napus) agroecosystems is needed. We examined the potential for SIGS in B. aeneus, via bud feeding, a field-relevant dsRNA exposure route. Oilseed rape buds were uniformly treated with dsRNA designed to target αCOP in B. aeneus. Our model control dsRNA (dsGFP) remained detectable on buds throughout the entire 3 d exposure period. When applied at 5 µg/µL, dsαCOP induced significant αCOP silencing 3 d after dietary exposure to buds treated with this dsαCOP concentration. We also observed a trend of increased αCOP silencing with increasing concentrations of dsαCOP at both 3 and 6 d. Furthermore, we observed a marginally significant and significant reduction in B. aeneus survival at 10 and 15 d, respectively. Our results suggest potential for developing a SIGS approach to B. aeneus management—though further experiments are needed to more fully understand this potential.

Floral biology and pollination strategy of Durio (Malvaceae) in Sarawak, Malaysian Borneo

Ng WS, Mohd-Azlan J, Wong SY. 2020. Floral biology and pollination strategy of Durio (Malvaceae) in Sarawak, Malaysian Borneo. Biodiversitas 21: 5579-5594. This study was carried out to investigate on the flowering mechanisms of four Durio species in Sarawak. The anthesis started in the afternoon (D. graveolens and D. zibethinus), evening (D. kutejensis) or midnight (D. griffithii); and lasted between 11.5 hours (D. griffithii) to 20 hours (D. graveolens). All four Durio species are generalists. Individuals of a fruit bat (Eonycteris spelaea, Pteropodidae) are considered as the main pollinator for D. graveolens, D. kutejensis, and D. zibethinus while spiderhunter (Arachnothera, Nectariniidae) is also proposed as a primary pollinator for D. kutejensis. Five invertebrate taxa were observed as secondary or inadvertent pollinators of Durio spp.: honeybee, Apis sp. (Apidae), stingless bee, Tetrigona sp. (Apidae), nocturnal wasp, Provespa sp. (Vespidae), pollen beetle (Nitidulidae), and thrip (Thysanoptera). Honey bees and stingless bees pollinated all four Durio species. Pollen beetles were found to pollinate D. griffithii and D. graveolens while nocturnal wasps were found to pollinate D. kutejensis and D. zibethinus. Thrips were found to pollinate D. griffithii only. Floral rewards include nectar, pollen and staminodes. The nectar production of D. graveolens, D. kutejensis, and D. zibethinus increased from the start of anthesis until just after midnight but decreased from then onwards. Durio griffithii produced consistent nectar concentration with inconsistent volume. Durio graveolens, D. griffithii, and D. zibethinus are partially self-incompatible.