The invasive tree Solanum mauritianum Scopoli remains one of the world's most widespread environmental weeds. Despite biocontrol providing one of the few viable long-term solutions to tackling S. mauritianum invasions globally, only South Africa and, more recently, New Zealand, have programmes in place. Ongoing biocontrol efforts against S. mauritianum are reviewed here with particular reference to South Africa. The South African programme has suffered a troubled history, with considerable research efforts culminating in the eventual release and establishment of only two insect agents, Gargaphia decoris Drake and Anthonomus santacruzi Hustache. The difficulties experienced have hindered research into new agents, causing apprehension in using biocontrol internationally. However, recent studies have demonstrated that biocontrol may be deserving of renewed investment, particularly within an integrated management context. In this review, we advocate for the revival of the S. mauritianum biocontrol programme in South Africa, and discuss possible avenues for future research internationally.
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.
h i g h l i g h t sGargaphia decoris herbivory reduced photosynthesis of Solanum mauritianum in both shaded and fullsun conditions. Photosynthesis declined due to chlorophyll removal and, indirectly, through physiological impairment. Herbivory rates were greater in the shade but feeding was more damaging in sun plants. Given the agent's potential effectiveness for biological control, factors constraining its performance need to be addressed.
Mpumalanga provinces (Terblanche et al., 2016). Parthenium hysterophorus is capable of growing in a multitude of habitats, maturing rapidly to form dense stands and extensive seedbanks, which aggressively outcompete and chemically inhibit native flora (Dhileepan and McFayden, 2012). The weed further prompts concern given its detrimental effects to agriculture (crop yield), animal husbandry and human health (Adkins and Shabbir, 2014). The annual nature (ephemeral) and extensive seedling recruitment of P. hysterophorus, has meant that mechanical and chemical clearing procedures are largely not feasible, ineffective and/or unsustainable. Thus, following prior successes in Australia, biological control efforts were initiated in South Africa during 2003, as one of the long-term solutions to managing P. hysterophorus invasion (Strathie et al., 2005(Strathie et al., , 2011.Considered one of the more promising biological control agents, the leaf-feeding beetle Zygogramma bicolorata Pallister (Coleoptera: Chrysomelidae) (Strathie et al., 2005), was released at various densely invaded sites throughout KwaZulu-Natal and Mpumalanga from 2013 onwards (Strathie et al., 2014;McConnachie, 2015). Both adult beetles and their larvae feed extensively on P. hysterophorus leaves, displaying a preference for those which are recently developed. Feeding by Z. bicolorata occurs continually from spring to autumn, resulting in severe, if not complete, defoliation of attacked leaves and the subsequent skeletonisation of whole plants (Dhileepan et al., 2000;McConnachie, 2015). These beetles hold great promise given their high fecundity (∼45 eggs) rapid development (egg to adult in ∼6 weeks) and longevity of up to 2 years (Dhileepan et al., 2000;Chidawanyika et al., 2017). During late autumn (April-May) Z. bicolorata enters a facultative soil diapause in response to decreasing temperatures, rainfall and photoperiod, re-emerging with the onset of favourable conditions during spring (October) (Strathie et al., 2011). The relatively recent release of Z. bicolorata in South Africa has meant the agent's efficacy remains under investigation, but ongoing studies highlight successful establishment, with variable incidence (Strathie et al., 2016). Despite the relatively recent establishment of Z. bicolorata in South Africa, extensively damaging outbreaks have been recorded (Chidawanyika et al., 2017).Folivory (feeding on leaf tissue) directly reduces leaf area, disrupting leaf function and ultimately altering the photosynthetic capacity (Zangerl et al., 2002). However, the photosynthetic impact of folivory cannot be predicted based on the amount of tissue removed, with studies showing increased, decreased or unchanged photosynthetic rates, all in response to folivory (Turnbull et al., 2007;Delaney, 2008;Kant et al., 2015). Although it is widely accepted that plants are well adapted to tolerate partial or complete defoliation (Peterson et al., 2004), recent studies suggest that photosynthetic reduction may be more common than previously thought (Delan...
It is generally suggested that invasive alien plant species (IAPS) negatively affect livelihoods. However, there is a need to find more generalisable patterns of IAPS impacts on livelihoods, which are useful for decision making at broad scales. Here we test the hypothesis that across South Africa poorer communities with high reliance on provisioning ecosystem services are more at risk from IAPS. To do so, we integrate two national-scale datasets; 1) The South African Plant Invaders Atlas, and 2) the National Census Data, which provides details on household income and selected provisioning ecosystem service use. Our analysis revealed that the potential impacts of IAPS increased as household income decreased, confirming that IAPS had greater negative effects in poorer landscapes. Furthermore, IAPS had the potential to be more impactful on selected provisioning ecosystem services where households were more reliant on them. Taken together, our results confirm that across South Africa, landscapes with poorer households and high reliance on the selected provisioning ecosystem services are likely the most negatively affected by IAPS. This novel broad-scale approach revealed landscape-scale patterns of the potential socio-ecological impacts of IAPS and will help decision makers direct resources for IAPS management to where they may be most effective.
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Insect declines have been attributed to several drivers such as habitat loss, climate change, invasive alien species and insecticides. However, in the global context, these effects remain patchy, whereas insect losses appear to be consistent worldwide. Increases in atmospheric CO2 concentrations are known to have indirect effects on herbivorous insects, but the effects on other insects are largely unexplored. We wondered if elevated atmospheric CO2 (eCO2) could influence the growth and survival of insects, not via rising temperature, nor through their changes in food quality, but by other means. Rearing tunnelling dung beetle Euoniticellus intermedius (Reiche, 1848) at pre‐industrial (250 parts per million [ppm]), current (400 ppm) and eCO2 levels (600 and 800 ppm), we found that exposure to eCO2 resulted in longer developmental times and increased mortality. Elevated CO2 also caused reduction of adult size and mass which is detrimental to dung beetle fitness. Additional results showed associated increases in CO2 levels inside dung brood balls, dung pH and respiration rates of the soil surrounding the developing dung beetles (CO2 flux). We thus hypothesize that elevated CO2 increases competition for O2 and nutrients between soil microbiota and subterranean insects. Given that many insect orders spend at least part of their life underground, our findings indicate the possibility of a negative ubiquitous effect of eCO2 on a large portion of the earth's insect biota. These findings therefore suggest an important area for future research on the soil community in the context of atmospheric change.
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