We all know that insecticides are used to control insects, and fungicides are used to control fungal infections. But both these chemicals are often sprayed on the same crops, so what effects do the fungicides have on insects? New research being undertaken as part of the Australian Grains Pest Innovation Program (AGPIP), is investigating this question.
The fight against aphids
In Australia, farmers devote substantial time and resources to protect crops from both the direct feeding damage of aphids and the transmission of viruses such as the barley yellow dwarf virus. Damage incurred can reduce yields by 6% from direct feeding and up to 30% from the viruses they transmit.
The Bird cherry-oat aphid (Rhopalosiphum padi) primarily attacks oats and wheat, but is also found on cereals, grass-based pastures, and corn. Strategies to control this species are heavily reliant on insecticides.
While these insecticide applications can be effective with good application practices, the propensity of aphids to evolve insecticide resistance adds another level of difficulty to an already challenging task and renders the species suitable for the study.
Importance of endosymbionts
Several fungicides have sublethal effects on arthropods via impacting the bacterial microbiome. This microbiome includes endosymbionts – a diverse group of bacteria that are hosted within the cells of aphids and are passed from parent to offspring.
Researchers have long known that endosymbionts are critical for aphid survival, but in recent times we are beginning to understand how endosymbionts shape the damage aphids inflict on crops. For instance, aphids can attack different crop types depending on their own endosymbionts.
Previous studies show that aphids exposed to certain chemicals subsequently produce poor-quality offspring. In some cases, it is argued that such across-generation effects are, in part, mediated by aphid endosymbionts because these bacteria are passed from parent to offspring. With this in mind, this study sought to explore whether endosymbionts mediate any impacts fungicides have on aphids within and across generations.
Fungicide’s impact on aphids
In collaboration with the University of Melbourne, Cesar Australia researchers examined the impacts of three commonly used fungicides on Bird cherry-oat aphid carrying both Buchnera (primary endosymbiont) and Rickettsiella (secondary endosymbiont) over multiple generations.
The three tested fungicides included chlorothalonil (Bravo Weather Stik®), pyraclostrobin (Cabrio®), and trifloxystrobin (Flint®). Five diluted concentrations ranging between 0.01 and 100x the field rate of the three fungicides were introduced to populations of Bird cherry-oat aphid.
The impacts (i.e., survival, lifespan, and reproductive ability) of these fungicides and the density of endosymbionts across multiple generations were conducted.
What did we find?
It was found that chlorothalonil (Bravo Weather Stik®) didn’t immediately affect aphid survival, although the following generation showed a substantially shorter lifespan and reduced reproductive ability.
Both pyraclostrobin (Cabrio®) and trifloxystrobin (Flint®) decreased aphid survival 48 hours after exposure. The trifloxystrobin also reduced the lifespan and reproductive ability of the following generation, while the pyraclostrobin did not affect these traits.
The study results also showed that exposure to these fungicides did not consistently impact the density of endosymbionts, Buchnera or Rickettsiella, suggesting the impacts observed were due to the insecticidal qualities of the fungicides.
Take home message
The fungicides tested negatively impacted Bird cherry-oat aphid, although interestingly, these impacts are not fully recognised until the generation after fungicide exposure. Although the fungicide impacts were not found to be influenced by endosymbionts, they may have some application in the management of aphid species. For example, suppose a given pesticide has negative impacts on aphids that persist over multiple generations. In that case, growers may require fewer pesticide applications to suppress aphid populations below an economic threshold.
Fewer pesticide applications may also reduce selection for resistance in pest populations. However, further research (e.g., how do fungicides impact beneficial insects?) is required before we can recommend any use of fungicides in aphid management. Nonetheless, our research findings provide a promising early step towards diversifying how growers can control aphids.
Find more information on insecticide resistance and aphid management strategies via the GRDC website.
The study discussed was supported by funding from the Grains Research and Development Corporation. We thank BASF Australia, Bayer CropScience, and Syngenta Australia for providing the fungicide samples tested in this study. We also thank The Grains Innovation Park for providing the R. padi line used in the study.