Plasticity in induced resistance to sequential attack by multiple herbivores in Brassica nigra

In nature, plants interact with multiple insect herbivores that may arrive simultaneously or sequentially. There is extensive knowledge on how plants defend themselves against single or dual attack. However, we lack information on how plants defend against the attack of multiple herbivores that arrive sequentially. In this study, we investigated whether Brassica nigra L. plants are able to defend themselves against caterpillars of the late-arriving herbivore Plutella xylostella L., when plants had been previously exposed to sequential attack by four other herbivores (P. xylostella, Athalia rosae, Myzus persicae and Brevicoryne brassicae). We manipulated the order of arrival and the history of attack by four herbivores to investigate which patterns in sequential herbivory determine resistance against the fifth attacker. We recorded that history of sequential herbivore attack differentially affected the capability of B. nigra plants to defend themselves against caterpillars of P. xylostella. Caterpillars gained less weight on plants attacked by a sequence of four episodes of attack by P. xylostella compared to performance on plants that were not previously damaged by herbivores. The number of times the plant was attacked by herbivores of the same feeding guild, the identity of the first attacker, the identity and the guild of the last attacker as well as the order of attackers within the sequence of multiple herbivores influenced the growth of the subsequent herbivory. In conclusion, this study shows that history of sequential attack is an important factor determining plant resistance to herbivores.

Effects of Functional Depletion of Doublesex on Male Development in the Sawfly, Athalia rosae

The doublesex (dsx) gene, which encodes a transcription factor, regulates sexual differentiation in insects. Sex-specific splicing of dsx occurs to yield male- and female-specific isoforms, which promote male and female development, respectively. Thus, functional disruption of dsx leads to an intersexual phenotype in both sexes. We previously identified a dsx ortholog in the sawfly, Athalia rosae. Similar to dsx in other insects, dsx in the sawfly yields different isoforms in males and females as a result of alternative splicing. The sawfly exploits a haplodiploid mode of reproduction, in which fertilized eggs develop into diploid females, whereas unfertilized eggs parthenogenetically develop into haploid males. In the present study, we knocked down the A. rosae ortholog of dsx (Ardsx) during several developmental stages with repeated double-stranded RNA (dsRNA) injections. Knockdown of Ardsx via parental RNA interference (RNAi), which enables knockdown of genes in offspring embryos, led to a lack of internal and external genitalia in haploid male progeny. Additional injection of dsRNA targeting Ardsx in these animals caused almost complete male-to-female sex reversal, but the resulting eggs were infertile. Notably, the same knockdown approach using diploid males obtained by sib-crossing caused complete male-to-female sex reversal; they were morphologically and behaviorally females. The same RNAi treatment did not affect female differentiation. These results indicate that dsx in the sawfly is essential for male development and its depletion caused complete male-to-female sex reversal. This is the first demonstration of functional depletion of dsx not causing intersexuality but inducing total sex reversal in males instead.

Survival of the Sawfly Athalia rosae Upon Infection by an Entomopathogenic Fungus and in Relation to Clerodanoid Uptake

Larvae of the turnip sawfly Athalia rosae are a pest of Brassicacae plants, as their feeding can cause defoliation of various crops of economic importance. The larvae and the adults of this sawfly species are known to take up different classes of chemical compounds from their respective host plants, with potentially deterrent functions against predators. In addition, compounds taken up by the adults, the clerodanoids, are known for their antimicrobial activity. These features could be a challenge to biocontrol strategies. Several natural enemies of A. rosae have been identified, targeting larval and pupal stages of A. rosae, which could potentially be used as biocontrol agents. However, targeting the adult stage of a larval pest in addition to targeting the juvenile stages may improve population control. In this study, we ask whether a strain of the entomopathogenic fungus Beauveria bassiana shows biological activity against A. rosae adults. We also investigate whether the behavior of clerodanoid uptake by the adults, which is commonly found, affects their survival in response to a B. bassiana exposure. We found a clear dose-response relationship, i.e., with increasing fungal conidia concentrations survival of A. rosae decreased. However, there was only a low incidence of mycelial growth and sporulation from A. rosae cadavers, indicating that either the fungus is not successfully developing inside this host, or it is not able to re-emerge from it. Clerodanoid uptake decreased the survival of healthy adults; however, it did not increase their survival to B. bassiana. Our results revealed that this strain of B. bassiana if applied alone is probably not suitable for biocontrol of this sawfly species, because A. rosae showed a high baseline resistance against this fungus. The behavior of clerodanoid uptake is unlikely to have evolved as a defense against this entomopathogenic fungus.

Consequences of pharmacophagous uptake from plants and conspecifics in a sawfly elucidated using chemical and molecular techniques

Pharmacophagy involves the sequestration of specialised plant metabolites for non-nutritive purposes and commonly occurs in insects. Here we investigate pharmacophagy in the turnip sawfly, Athalia rosae, where adults not only collect specialised metabolites (clerodanoids) from a plant (Ajuga reptans), but also from the exterior of conspecifics via fighting. Using behavioural assays, chemical analytics, and RNAseq we show that when individuals nibble on conspecifics that have already acquired clerodanoids from A. reptans leaves, this nibbling results in the transfer of compounds between individuals. Furthermore, unlike other pharmacophagous insects, the acquisition of clerodanoids by A. rosae from the leaves of A. reptans does not induce the upregulation of known detoxification or sequestration genes and pathways. In contrast, pharmacophagous nibbling on conspecifics results in the upregulation of metabolic pathways associated with elevated metabolic rates and increased energy consumption. It therefore seems that individuals attack conspecifics to acquire clerodanoids despite the apparent metabolic costs of this form of pharmacophagy compared to clerodanoid uptake from a plant. Changes in the metabolic phenotype of A.rosae individuals consequently has profound consequences for social interactions with possible ramifications for their social niche.Summary statementThe turnip sawfly (Athalia rosae) gains potentially beneficial compounds from the leaf surface of non-food plants (e.g. Ajuga reptans), but can also steal these compounds from conspecifics via aggressive nibbling.

Фітосанітарний стан посівів ріпаку ярого в умовах північно-східного лісостепу України

Динаміку розповсюдженості шкідників ріпаку ярого вивчено в умовах північно-східного Лісостепу України у 2005‒2018 рр. Методика досліджень була загальноприйнятою. У посівах ріпаку ярого найбільш розповсюдженими шкідниками були квіткоїд ріпаковий (Meligethes aeneus F.), блішки хрестоцвіті (Phyllotreta spp.) та пильщик ріпаковий (Athalia rosae L.). Блішки заселяли 100 % площ хрестоцвітих у фазу сходів культури. На посівах ріпаку блішки пошкодили 12,0 ‒ 35,0 % рослин. Найбільша пошкодженість шкідниками становила 67,0 % у 2006 р. і 66,0 % – у 2005 р. Чисельність блішок на хрестоцвітих складала 3,0‒5,0 екз./м2. Найвищою(18,0 екз./м2) вона була у 2006 р. Квіткоїд ріпаковий заселяв 100,0 % посівів, а у 2006 р. – 85,0 %, у 2010 р. – 91,0 % посівів у фазу бутонізації‒цвітіння. У посівах ріпаку ярого квіткоїд пошкодив 17,0‒37,0 % рослин. Найбільша пошкодженість шкідниками була у 2006 р. і складала 74,0 %, у 2005 р. ‒ 69,0 %, у 2007 р. – 63,0 %. Чисельність квіткоїда ріпакового становила у середньому 2,0‒6,0 екз./рослину. Найвищою вона була у 2006 р. (30,0 екз./рослину). Розповсюдженість пильщика ріпакового була найменшою зі спеціалізованих шкідників ріпаку ярого. Він заселяв 100,0 % посівів лише у 2005 р., а у 2008 р. – тільки 64,0 % посівів. За роки досліджень пильщик ріпаковий заселяв, в основному, в межах 14,0‒50,0 % посівів. Комахами було пошкоджено 3,0‒8,0 % рослин. Чисельність пильщика ріпакового становила 0,9‒2,0 екз./рослину. Найвища чисельність шкідників 7,0 екз./рослину була у 2006 р., дещо меншою ‒ у 2007 р. (4,0 екз./рослину). В Сумській області за період 2005‒2018 рр. площі посівів ріпаку ярого змінювалися в межах 1,1‒22,2 тис. га, валовий збір насіння – 1,9‒22,3 тис. т. Заходи захисту посівів ріпаку ярого від шкідників включають профілактичні і винищувальні методи. Для захисту посівів від блішок хрестоцвітих у фазу сходів використовують інсектицидні протруйники. У період вегетації проти блішок хрестоцвітих (3‒5 жуків на м2), квіткоїда ріпакового (5‒6 жуків на рослину), ріпакового пильщика посіви обприскують інсектицидами.