Physiological response of Parthenium hysterophorus to defoliation by the leaf-feeding beetle Zygogramma bicolorata

The annual shrub, Parthenium hysterophorus L. (Asteraceae), is a severe noxious weed of global significance, having been the target of control programmes in several affected countries. Biological control efforts in South Africa have seen a total of four agents released to date, one of the most promising of which is the leaf-feeding beetle, Zygogramma bicolorata Pallister (Coleoptera: Chrysomelidae), first released in 2013. Detailed study of the ecophysiological interactions of insect herbivores and their target weeds are under-represented in the biological control literature. Extensive leaf-feeding by Z. bicolorata adults metabolically impaired P. hysterophorus leaves, resulting in a ∼36% reduction to photosynthesis in remaining leaf tissue. However, in response to Z. bicolorata herbivory, adjacent undamaged leaves showed an up-regulation in photosynthesis of ∼11%. Photosynthetic reduction was attributed to the removal of leaf tissue, physiological impairment and increased microbial abundance resulting from Z. bicolorata. Zygogramma bicolorata was found to increase the number of bacterial and fungal microbes present on P. hysterophorus leaves. Despite the ability to photosynthetically up-regulate in response to herbivory, P. hysterophorus is unlikely to overcome the physiological implications associated with continued Z. bicolorata feeding and the increase in microbes. Given the potential effectiveness of Z. bicolorata as a biological control agent, efforts should be made to maximise incidence of the beetle in the field

Does host plant quality constrain the performance of the Parthenium beetle Zygogramma bicolorata?

In South Africa, the leaf-feeding beetle, Zygogramma bicolorata Pallister (Coleoptera: Chyrsomelidae), was released in 2013 against the invasive annual herb Parthenium hysterophorus L. (Asteraceae: Heliantheae). Poor field establishment and variable incidence of the beetle, during the first few years after release, have led to concerns surrounding potential constraints, including host plant quality. This study assessed the influence of high, medium and low host plant quality, as influenced by fertiliser application, on the survival, development and fecundity of Z. bicolorata. Although egg to adult survival was high (∼80%) and did not differ between plant quality treatments, larvae developed fastest on plants of high and medium quality. Larval feeding was highly damaging, reducing the growth, reproduction and biomass of P. hysterophorus in all three plant quality treatments. Fecundity was associated with plant quality, with larger, more fecund females developing on higher quality plants. Overall, this research may aid current mass-rearing efforts, further field research to verify these findings and direct the selection of more suitable release sites for Z. bicolorata in South Africa

Parthenium avoids drought: understanding the morphological and physiological responses of the invasive herb Parthenium hysterophorus to progressive water stress

Parthenium hysterophorus L. is a noxious annual invasive herb which threatens biodiversity, food security and human and animal health in various tropical and sub-tropical regions globally. Given that these regions, including South Africa, experience frequent drought events, it is important that the invasive potential of this weed be better understood in the context of water stress. This study aimed to assess the physiological and morphological responses of P. hysterophorus to water stress. To elicit these responses, juvenile and adult plants were progressively water stressed over a six-week period. Despite significant reductions in leaf water potential, relative leaf water content and stomatal conductance from severe soil water stress, plants maintained positive net photosynthesis, albeit at a severely reduced rate. Concomitantly, water-use efficiency of P. hysterophorus increased by more than 300%. Morphologically, water-stressed plants exhibited reductions in leaf area and leaf number, with adults accelerating flower production when compared to well-watered plants. Linked to this, were alterations in biomass, with juveniles and adults reducing above-ground biomass by 75% and 35%, respectively. Overall, many of the physiological and morphological responses are indicative of a largely drought avoidance strategy, allowing P. hysterophorus to persist during periods of water-stress as leafy basal rosettes (juveniles) or as reproductive herbs (adults). This research yielded valuable insights into the underlying water relations of P. hysterophorus, and how these may relate to the potential spread, detriment and management of the weed in various sub-tropical environments

Cryptic diversity, molecular phylogeny and biogeography of the rock- and leaf litter-dwelling land snails of Belau (Republic of Palau, Oceania)

The endemic diplommatinid land snails (Caenogastropoda: Mollusca) of Belau (Republic of Palau, Micronesia) are an exceptionally diverse group of largely undescribed species distributed among rock and leaf litter habitats on most of Belau's 586 islands. Diplommatinid shell morphology (e.g. shell sculpture) reflects habitat type. In this study, I analysed a subset of the 90 diplommatinid species representing a broad geographical spread of islands in order to reveal the species' phylogenetic relationships and biogeography within the Belau archipelago. Diplommatinid species from the islands of Yap, Pohnpei, Kosrae and Guam are also included in the analysis. One nuclear (28S rRNA) and two mitochondrial (16S rRNA, COI) gene regions comprising 1906 bp were used for phylogenetic reconstruction. Results show that (i) the Belau Diplommatinidae are not monophyletic, as Guam and Yap species should be included as part of the radiation, (ii) Pohnpei and Kosrae species are highly divergent from Belau diplommatinids, (iii) there is little evidence for in situ radiation within individual Belau islands, (iv) spined and heavily calcified rock-dwelling species form a well-supported clade, and (v) Belau diplommatinid genera are in need of revision