Insect herbivores have the potential to devastate certain plant species, a trait that scientists have cleverly exploited towards the control and management of unwanted and problematic invasive plants. Termed “biological control” or “biocontrol,” this ecological practice aims to suppress invasive species through the importation and release of their specific natural enemies or predators . However, the success of these biological control efforts may be affected by a variety of abiotic and biotic factors, most interestingly by the target plant’s response to herbivory.
Numerous plant responses are known to be induced by herbivory and are broadly grouped as defensive or compensatory. Defensive responses seek to prevent or deter herbivory and include traits such as leaf hairs or secondary leaf compounds. Whereas compensatory responses attempt to mitigate or overcome the losses incurred by herbivory and often involve adjustments to the plants physiological, namely photosynthetic, processes .
Originating from Central and South America, the annual shrub Parthenium hysterophorus (Parthenium) is an invasive weed of global significance. The weed has severe negative socioeconomic and ecological impacts, including threats to agriculture, food security, human health, and biodiversity. Given these threats, many countries are electing to use biological control as an environmentally safe, cost-effective, and sustainable way to manage Parthenium invasions .
In South Africa, a total of four biological control agents have been released to date: a leaf-rust fungus and three insects . The most promising of these insect agents has been the leaf-feeding beetle Zygogramma bicolorata, first released in 2013. Although recent efforts in South Africa are commended, the beetle Z. bicolorata has not yet reached its full potential, prompting investigation towards understanding and improving its effectiveness.
Within this context, our research assessed the physiological responses of Parthenium to leaf-feeding by its biocontrol beetle Z. bicolorata. The study further investigated the mechanisms by which these beetles may reduce the photosynthesis of Parthenium, with a focus on microbial interactions. A variety of natural and simulated herbivory experiments, using gas exchange and microbial sampling, were performed to investigate this.
Our results showed that the beetles were able to reduce the photosynthetic output of damaged Parthenium leaves by 36%. Interestingly, this reduction was not due to the removal of leaf tissue, as suspected, but rather the introduction of microbes by Z. bicolorata. High numbers of bacterial and fungal microbes were found on beetle herbivorized leaves, likely originating from the mouthparts and frass deposits of Z. bicolorata. This suggests that microbes contribute a great deal towards the biological control of Parthenium, and may be equally, if not more damaging, than the removal of leaf tissue itself.
In addition to decreasing Parthenium’s productivity (photosynthesis), the beetles leaf-feeding also impaired the plants’ metabolism. These metabolic impairments reduced the water- and nutrient-use efficiency of Parthenium, weakening the plant and making it more susceptible to both herbivory and water-stress.
Parthenium attempted to compensate for beetle herbivory by up-regulating photosynthesis in undamaged leaves by 11%. Although this up-regulation is indeed impressive, it is partial and unable to overcome the losses and physiological stresses associated with continued herbivory by the beetles. Furthermore, these beetles may mitigate the photosynthetic up-regulation of Parthenium by defoliating approximately a third of the plant.
This research showcases the potential of Z. bicolorata to be a highly damaging insect, despite attempts by Parthenium to compensate for its herbivory. Given the biological control potential of Z. bicolorata, research efforts should be continued to increase beetle numbers and damage in the field.
This work is of significance as it highlights the effectiveness that biological control offers towards the control of problematic invasive species like Parthenium. The research also advocates for the use of ecophysiological assessments as a tool to better understand and improve current and future biological control endeavors.
These findings are described in the article entitled Physiological response of Parthenium hysterophorus to defoliation by the leaf-feeding beetle Zygogramma bicolorata, recently published in the journal Biological Control. This work was led by Blair Cowie from the University of the Witwatersrand, Johannesburg, South Africa.
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