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

An approach to the development of a national strategy for controlling invasive alien plant species: the case of parthenium hysterophorus in South Africa

Invasive alien species require co-ordinated management if negative impacts are to be effectively avoided. Here we describe a strategy for the management of Parthenium hysterophorus L.(Asteraceae) in South Africa. P.Hysterophorus is an annual herb native to tropical America, which has become invasive in over 30 countries. The strategy sets goals for (1) the prevention of spread to new areas; (2) local eradication of isolated populations; (3) containment in areas where eradication is not possible; and (4) actions to protect assets where containment is no longer an option. We developed both a national strategy to set policy and to monitor progress towards goals at a national level and an implementation plan to set goals and timeframes for their achievement at local levels. It is not clear, at this stage, whether of not the goals of the strategy are achievable because implementation will face many challenges arising from ecological features of the target plant, social and cultural practices that will influence management, inadequate levels of funding and multiple political considerations. Our strategy proposes regular assessment using high-level indicators, a practice that is widely recognised as essential but seldom implemented at a national scale. Because the outcomes are uncertain, it is vital that regular monitoring of outcomes should be instituted from the start, so that both appropriate adjustments can be made to the strategy and lessons for the implementation of similar strategies elsewhere can be derived