Adaptation of flea beetles to Brassicaceae: host plant associations and geographic distribution of Psylliodes Latreille and Phyllotreta Chevrolat (Coleoptera, Chrysomelidae)

Abstract

The cosmopolitan flea beetle genera Phyllotreta and Psylliodes (Galerucinae, Alticini) are mainly associated with host plants in the family Brassicaceae and include economically important pests of crucifer crops. In this review, the host plant associations and geographical distributions of known species in these genera are summarised from the literature, and their proposed phylogenetic relationships to other Alticini analysed from published molecular phylogenetic studies of Galerucinae. Almost all Phyllotreta species are specialised on Brassicaceae and related plant families in the order Brassicales, whereas Psylliodes species are associated with host plants in approximately 24 different plant families, and 50% are specialised to feed on Brassicaceae. The current knowledge on how Phyllotreta and Psylliodes are adapted to the characteristic chemical defence in Brassicaceae is reviewed. Based on our findings we postulate that Phyllotreta and Psylliodes colonised Brassicaceae independently from each other.

Keywords: Alticini; chemical plant defence; detoxification; glucosinolates; plant-insect interaction; secondary plant metabolites; sequestration.

Figure 1.

Host plant associations of the genera Psylliodes (A) and Phyllotreta (B). The host plants of 107 Psylliodes species and 117 Phyllotreta species have been reported in the literature. The numbers of species which feed on plants in one plant family (monophagous and oligophagous), and the number of polyphagous species are given as percentages. 18% of the Phyllotreta species feed on more than one family in the order Brassicales (Brassic., Brassicaceae; Cappar., Capparaceae; Cleom., Cleomaceae; Resed., Resedaceae; Tropaeol., Tropaeolaceae). For detailed information, refer to Suppl. material 1 (Psylliodes) and 3 (Phyllotreta).

Figure 2.

Distribution of 207 Psylliodes species in the different zoogeographical regions (A), and host plant associations of all species (As) and endemic species (Es) for each zoogeographical region (B). For detailed information, refer to Suppl. material 1.

Figure 3.

Distribution of 242 Phyllotreta species in the different zoogeographical regions (A), and host plant associations of all species (As) and endemic species (Es) for each zoogeographical region (B). For detailed information, refer to Suppl. material 3.

Figure 4.

Metabolism of glucosinolates in Psylliodeschrysocephala and Phyllotretastriolata. Upon herbivory, glucosinolates are usually hydrolysed by the plant enzyme myrosinase to an unstable aglucone, which spontaneously rearranges to a toxic isothiocyanate. In the presence of plant specifier proteins, other hydrolysis products such as thiocyanates and nitriles are formed. Both flea beetle species sequester glucosinolates in their bodies, suggesting that not all glucosinolates are hydrolysed in feeding-damaged plant tissue. Sequestered glucosinolates may be activated for defensive purposes by an insect myrosinase in Ph.striolata, but not in Ps.chrysocephala. In addition, Ps.chrysocephala partially detoxifies glucosinolates by desulfation, whereas no glucosinolate sulfatase activity was found in Ph.striolata. According to a quantitative feeding study performed with Ps.chrysocephala, most ingested glucosinolates are activated, and isothiocyanates are detoxified by conjugation to glutathione. The isothiocyanate-glutathione conjugate is metabolized via the mercapturic acid pathway to several cyclic metabolites in Ps.chrysocephala adults (Beran et al. 2018). Examples of three structurally different glucosinolate side-chains are shown in the box. Beetle photos: Anna Schroll.