We all love a ladybird beetle. They are bright, fun and cosmopolitan. They also have ravenous appetites, with soft bodied insects being a particular preference. If you’re an unsuspecting aphid, beware.
Predatory insects, such as our ladybird beetle friend, can be found consuming insects in both natural and anthropogenic ecosystems. Growers recognise the advantage of in-crop natural enemy populations. Their presence is indicative of a thriving ecosystem, plus why pay for pest control services when you can have them for free? Ensuring that farming practices are benign (or beneficial) towards natural enemy populations is a crucial element of Integrated Pest Management (IPM).
Growers can support natural enemy populations by understanding their life cycles and nutritional needs, and planting belts of alternate food sources such as nectar-rich plant species, which can also act as a place of shelter. Reducing the potential for non-target impacts of pesticides on natural enemy species, through strategic spraying of borders or affected patches, or application of highly specific chemical options, are also important aspects of preserving these species.
The effectiveness of natural enemies
Natural enemies can be specialists, the fussy eaters of the insect world that consume or parasitise a narrow range of prey species, or generalists, which will happily feed on a variety of different insects and other food types, such as detritus.
The capacity of a natural enemy population to control pest numbers varies between species.
For instance, a 2018 European study of predation rates of three ladybird beetle species – Adalia bipunctata, Hippodamia variegata and Scymnus interruptus – on green peach aphid (Myzus persicae) in greenhouse-grown sweet pepper crops found significant variation in pest control effects between ladybird species (Beltrà et al. 2018).
Effectiveness of control would also be influenced by other factors such as the pest in question, farming practices, the surrounding environment and ecological factors.
However, there are large research gaps in relation to gaining a better understanding of how effective natural enemy species may be at suppressing pest populations, their biology and phenology, and how their effectiveness is influenced by environment factors – including some you wouldn’t even consider.
Natural enemies in grains
Among grain crops, there is a diversity of insect natural enemies that can contribute to the control and suppression of arthropod pest populations. This includes predatory invertebrates – such as hoverfly larvae, lacewings, predatory bugs, ants, ground dwelling and transient predatory beetles, predatory mites, and spiders – as well as a range of transient parasitoid wasp species, such as Diaeretiella rapae and Aphidius sp, that are found naturally in grain crops (although you do need to look hard).
The large scale of farming operations in the grains industry makes release of commercially reared natural enemies less feasible than in horticultural systems. Other factors limiting on-farm stewardship of natural enemies is the lack of research regarding the pest suppression capacity of these species in grain systems and the impact of pesticide products on natural enemy species.
The degree of the potential impact of a pesticide on natural enemy populations depends on a variety of factors, including the toxicity and sublethal effects of the chemical to the species.
Understanding toxicity impacts of pesticide contact for natural enemy species is a key element in helping growers pick chemical products that are less disruptive to these populations. However, determining sub-lethal impacts of pesticides, such as establishing if pesticide exposure can affect reproduction and therefore the survival of the next generation, is also critical.
Filling knowledge gaps
At this point in time a synthesis of the effects of different registered pesticides on key natural enemy species relevant to the Australian grains industry has not yet been made available for growers. While a natural enemy toxicity guide developed as part of the cotton industry’s broader pest management guide is useful in providing guidance on toxicity, differences in field application rates, registered products and natural enemy species between industries limits the applicability of this resource for the grains industry.
Working as part of the Australian Grains Pest Innovation Program (AGPIP), cesar aims to fill some of the knowledge gaps relating to pesticide toxicity to natural enemy species.
A review of research literature reporting toxicity rates for natural enemies relevant to the Australian grains industry has already been completed by the Research Scientist undertaking the natural enemy work, Kathy Overton. The review has helped to identify specific knowledge gaps relating to particular chemicals and natural enemies and has helped to inform the next stage of investigation – laboratory-based trials. Kathy is currently working to fill the knowledge gaps by undertaking laboratory bioassays and working with industry partners to gather additional data. This will support the development of a guide detailing insecticide toxicity ratings on natural enemies relevant to the grains industry, which will be released later next year. The guide will be updated over time, as new toxicity findings emerge.
This research is being undertaken as part of the Australian Grains Pest Innovation Program (AGPIP). AGPIP is a collaboration between the Pest & Environmental Adaptation Research Group at the University of Melbourne and cesar. The program is a co-investment by the Grains Research and Development Corporation (GRDC) and the University of Melbourne, together with in-kind contributions from all program partners.
Thanks to Dr Paul Umina and Kathy Overton for contributions to this article.
This article draws on findings from:
Beltrà et al. 2018. Predation rate and performance of three ladybirds against the green peach aphid Myzus persicae in sweet pepper. Entomol Exp Appl. 166: 491-499.
Holloway et al. 2008. Management of beneficial invertebrates and their potential role in integrated pest management for Australian grain systems. Australian Journal of Experimental Agriculture. 48: 1531-1542.
Macfadyen et al. 2019. Identifying critical research gaps that limit control options for invertebrate pests in Australian grain production systems. Austral Entomology. 58: 9-26.