Genetics of Greenbug (Homoptera: Aphididae) Virulence to Resistance in Wheat and Sorghum

Entomolog

Tomato trichomes are deadly hurdles limiting the establishment of Amblyseius swirskii Athias-Henriot (Acari: Phytoseiidae)

[EN] Amblyseius swirskii is a predatory mite widely used for the control of very important pest species, such as whiteflies and thrips, in organic farming and conventional agriculture. However, this species cannot establish on tomato crops, probably due to the toxic effects of plant trichomes and their exudates. We evaluated tomato plants for effects on: a) A. swirskii preference mediated by plant volatiles, b) A. swirskii development, predation capacity and reproductive performance, c) the dispersal and survival of mites as affected by stem trichomes, and d) mite survival as a function of secondary metabolites secreted by tomato trichomes. The results showed that A. swirskii mites which gained experience foraging on tomato plants, tend to avoid them. The survival of A. swirskii eggs and juveniles on tomato leaves was not different from that on sweet pepper. However, adult survival was significantly lower when tested on whole plants. This was ostensibly due to the impact of trichomes and their secondary metabolites that are abundant on the stems and which negatively impacted mite dispersal on the plant. Among the secondary metabolites detected in tomato trichomes, the strongest negative effects were associated with acyl sugars. Acyl sugars were highly toxic to the mites and also, physically accumulated on their bodies after walking on tomato stems. Altogether, our results reveal why A. swirskii is not an efficient biocontrol agent on tomato crops.This work was funded by the European Union's Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement No 641456. JGC was supported by the Spanish Ministry of Economy and Competitiveness, Ramon y Cajal Program (RYC-2013-13834). JLR was supported by the Spanish Ministry of Economy and Competitiveness through a "Juan de la Cierva-Formacion" grant (FJCI-2016-28601). Mass spectrometric determinations were carried out at Servei Central d'Instrumentacio Cientifica (SCIC) from Universitat Jaume I.Paspati, A.; Rambla Nebot, JL.; López-Gresa, MP.; Arbona, V.; Gómez-Cadenas, A.; Granell Richart, A.; González-Cabrera, J.... (2021). Tomato trichomes are deadly hurdles limiting the establishment of Amblyseius swirskii Athias-Henriot (Acari: Phytoseiidae). Biological Control. 157:1-9. https://doi.org/10.1016/j.biocontrol.2021.104572S1915

Russian wheat aphids (Diuraphis noxia) in China: Native range expansion or recent introduction?

In this study, we explore the population genetics of the Russian wheat aphid (RWA) (Diuraphis noxia), one of the world’s most invasive agricultural pests, in north-western China. We have analysed the data of 10 microsatellite loci and mitochondrial sequences from 27 populations sampled over 2 years in China. The results confirm that the RWAs are holocyclic in China with high genetic diversity indicating widespread sexual reproduction. Distinct differences in microsatellite genetic diversity and distribution revealed clear geographic isolation between RWA populations in northern and southern Xinjiang, China, with gene flow interrupted across extensive desert regions. Despite frequent grain transportation from north to south in this region, little evidence for RWA translocation as a result of human agricultural activities was found. Consequently, frequent gene flow among northern populations most likely resulted from natural dispersal, potentially facilitated by wind currents. We also found evidence for the longterm existence and expansion of RWAs in China, despite local opinion that it is an exotic species only present in China since 1975. Our estimated date of RWA expansion throughout China coincides with the debut of wheat domestication and cultivation practices in western Asia in the Holocene. We conclude that western China represents the limit of the far eastern native range of this species. This study is the most comprehensive molecular genetic investigation of the RWA in its native range undertaken to date and provides valuable insights into the history of the association of this aphid with domesticated cereals and wild grasses

Insecticidal Activity of Some Reducing Sugars Against the Sweet Potato Whitefly, Bemisia tabaci, Biotype B

The effects of 16 sugars (arabinose, cellobiose, fructose, galactose, gentiobiose, glucose, inositol, lactose, maltose, mannitol (a sugar alcohol), mannose, melibiose, ribose, sorbitol, trehalose, and xylose) on sweet potato whitefly Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) survival were determined using in vitro bioassays. Of these sugars, arabinose, mannose, ribose, and xylose were strongly inhibitory to both nymphal and adult survival. When 10% mannose was added to the nymphal diet, 10.5%, 1.0%, and 0% developed to the 2nd, 3rd, and 4th instars, respectively. When 10% arabinose was added, 10.8% and 0% of the nymphs molted to the 2nd and 3rd instars, respectively. Addition of 10% xylose or ribose completely terminated B. tabaci development, preventing the molt to the 2nd instar. With decreasing sugar concentrations the inhibitory effect was significantly reduced. In tests using adults, arabinose, galactose, inositol, lactose, maltose, mannitol, mannose, melibiose, ribose, sorbitol, trehalose, and xylose significantly reduced mean day survival. Mortality rates were highest when arabinose, mannitol, mannose, ribose, or xylose was added to the diet. Mean day survival was less than 2 days when adults were fed on diet containing 10% of any one of these five sugars. When lower concentrations of sugars were used there was a decrease in mortality. Mode of action studies revealed that toxicity was not due to the inhibition of alpha glucosidase (converts sucrose to glucose and fructose) and/or trehalulose synthase (converts sucrose to trehalulose) activity. The result of agarose gel electrophoresis of RT-PCR products of bacterial endosymbionts amplified from RNA isolated from whiteflies fed with 10% arabinose, mannose, or xylose indicated that the concentration of endosymbionts in mycetomes was not affected by the toxic sugars. Experiments in which B. tabaci were fed on diets that contained radio-labeled sucrose, methionine or inulin and one or none (control) of the highly toxic sugars showed that radioactivity (expressed in DPM) in the body, in excreted honeydew and/or carbon dioxide, was significantly reduced as compared to controls. Thus, it appears that the ability of insecticidal sugars to act as antifeedants is responsible for their toxicity to B. tabaci

Baubles, Bangles, and Biotypes: A Critical Review of the use and Abuse of the Biotype Concept

Pest species of insects are notoriously prone to escape the weapons deployed in management efforts against them. This is particularly true in herbivorous insects. When a previously successful tactic fails the insect population has apparently adapted to it and is often considered to be a new or distinct entity, and given the non-formal category ‘biotype’. The entities falling under the umbrella term ‘biotype’ are not consistent either within or between biotypes, and their underlying genetic composition and origins, while generally unknown, are likely heterogeneous within and variable between biotypes. In some cases race or species may be more appropriate referents. Some examples of applications of the concept in the context of host plant resistance are discussed. It is argued here that the term ‘biotype’ and its applications are overly simplistic, confused, have not proved useful in current pest management, and lack predictive power for future management