Take home messages

• Augmentation offers a future IPM option: Preliminary findings suggest that parasitoid wasp augmentation may help reduce aphid populations in certain regions
• Re-evaluate broad-spectrum insecticide use: Broad-spectrum insecticides disrupt natural pest control by killing parasitoids and other beneficial insects
• Incorporate selective insecticides: Selective insecticides complement biological control efforts and support long-term IPM strategies
• Tailor pest management to regional needs: Recognise that different species respond differently to insecticides and local pest dynamics vary. Combine augmentation and selective chemicals based on regional needs.

Background

Aphids are major pests in grain crops, causing yield losses through feeding and virus transmission. Traditional management strategies rely heavily on broad-spectrum insecticides, which, while providing quick and effective control, inadvertently harm beneficial insects and disrupt ecological balances. This overreliance has also driven the emergence of insecticide resistant aphid populations, making future control increasingly challenging. Resistance in key species like green peach aphid (Myzus persicae) and blue green aphid (Acyrthosiphon kondoi) has significantly reduced the efficacy of many insecticides. This has heightened the risk of control failures and compelled growers to transition to more expensive, newer chemistries as older products fail.

Integrated pest management

Integrated pest management (IPM) promotes sustainable pest control by combining biological, cultural, and chemical methods to minimise harm to non-target species and reduce selection pressure for resistance evolution. Beneficial insects, particularly parasitoid wasps such as Diaeretiella rapae, Aphidius colemani, and Aphidius ervi, play a key role in biological control by suppressing aphid populations below economic thresholds. These species occur naturally in canola paddocks as resident  populations and can also be released strategically through augmentative biocontrol to enhance their impact. Augmentative biocontrol involves rearing and strategically releasing commercially produced beneficial insects, such as parasitoid wasps, to boost their population density and enhance pest suppression, particularly when resident populations are insufficient.

Knowledge gaps

Questions remain regarding the consistency of alternative methods in suppressing pests compared to conventional pesticides. While augmentation shows promise in small-scale cropping systems, its effectiveness in broadacre grains like canola remains uncertain.

Whilst laboratory studies and anecdotal evidence highlight the role of beneficial insects in pest control for grains, understanding their real-world efficacy is crucial for fostering confidence in IPM strategies. Targeted research into the efficiency of parasitoid wasps in suppressing aphid populations can provide growers with actionable insights.

Equally important is providing growers and advisers with tools to make IPM-aligned pesticide decisions. Although an important strategy, many insecticides are toxic to beneficial insects, particularly parasitoids. Judicious pesticide use, including the application of selective pesticides which target specific pests while minimising harm to beneficial insects, can enhance pest suppression by preserving resident beneficial populations. However, the grains industry has faced a shortage of science-backed data on insecticide toxicity to beneficial insects, complicating efforts to implement such practices effectively.

This paper addresses these gaps through two discrete but complementary studies. Study 1 presents findings from the recently updated beneficials chemical toxicity table, detailing the impacts of commonly used insecticides on beneficial insects, with a focus on parasitoids and their practical implications for the grains industry. Study 2, whilst preliminary, examines aphid control by comparing the population dynamics and efficacy of resident and augmented parasitoids in non-chemical settings versus paddocks treated with conventional pesticides with no parasitoid releases.

Methods

Study 1: Pesticide toxicity on aphid parasitoids

Generation of toxicity data

Key beneficial insects and insecticides relevant to the Australian grains industry were identified through expert consultation. Relevant to this paper, acute toxicity testing occurred for 20 insecticides on three economically important aphid parasitoids: D. rapae, A. colemani, and Aphelinus abdominalis (Overton et al. 2023). Of these 20 insecticides, 9 are currently registered for use in grain crops for control of aphids or other pests. Mortality ratings followed International Organisation for Biological Control (IOBC) guidelines for controlled laboratory conditions: low (<30%), medium (30–79%), high (80–99%), and very high (>99%).

Pre-existing data on insecticide toxicity to these beneficials were compiled via a systematic literature review, with laboratory tests conducted to address data gaps using standardised IOBC protocols. For more on this methodology and an easy-to-use table that summarises the impacts of insecticides on other beneficial groups in cereals, refer to Knapp et al. (2023) or the Beneficials Chemical Toxicity Table.

Study 2: Efficiency of aphid control by parasitoids

Canola field treatments

To evaluate the effectiveness of parasitoids (both natural and augmented) in controlling aphids compared to conventional insecticide-based management, we designed an experiment with three plot types:

• augmented biocontrol: introduced a commercially produced mix of three parasitoid species (released at 1 000 parasitoids per ha) to enhance local parasitoid populations
• natural biocontrol: assessed the impact of resident parasitoid populations on aphid control, including any indirect effects from the absence of insecticide applications
• conventional control: evaluated pest management using the grower’s standard practices, including typical insecticide types and application rates for invertebrate pest control.

All plots were 5ha. Forty-five experimental plots were established, consisting of 17 paired plots (two natural:conventional, two augmented:conventional, 13 augmented:natural), 12 unpaired natural plots, and 14 unpaired augmented plots. In paired plots, a set of two paddocks within a property were assigned experimental and conventional treatments to compare local outcomes. Unpaired plots, consisting of single paddocks with augmented or natural control, assessed regional trends.

Parasitoid releases and surveys

Parasitoids (D. rapae: 50%, A. colemani: 25%, A. ervi: 25%) were released in biodegradable capsules (500 parasitoids per capsule) at a rate of 1 000 parasitoids per hectare during July–August, aligning with warming temperatures and declining seed treatment efficacy.

Each 5-ha plot was surveyed across 10 subplots, recording aphid species, abundance, parasitised aphids (mummies), crop stage, and plant density (baseline only). Surveys were conducted in:

• winter (July–August): baseline assessment of aphids and parasitoids before releases
• spring (September–October): follow-up assessment coinciding with peak aphid and parasitoid activity

Data collection

Aphid and mummy counts were estimated in field. Mummies were collected for lab rearing at 20°C to identify parasitoid species, with unsuccessful rearing noted after two weeks. Aphids without visible parasitism were also reared when no mummies were found. Growers were asked to share yield and input cost data for each plot. With many sites harvested in November, these data are pending.

Results

Study 1: Pesticide toxicity on aphid parasitoids

The tested parasitoid species exhibited both shared and unique responses to different insecticides (Table 1), highlighting the importance of conducting species-specific toxicity assessments.

Table 1. IOBC toxicity ratings (1 = <30%, 2 = 30–79%, 3 = 80–99%, 4 = >99% mortality) based on mortalities observed at the maximum registered field rate (MRFR) in Australian grain crops at 48 and 72h after treatment (HAT) for D. rapae, A. colemani, and A. abdominalis for the nine chemicals currently registered for use in grain crops for control of aphids or other pests (Overton et al. 2023).

Unsurprisingly, broad-spectrum chemicals like methomyl and dimethoate caused 97–100% mortality across all three species, whilst in general, active ingredients promoted as selective, such as flonicamid and afidopyropen, caused low mortality (<30%), highlighting these as safer options for growers targeting aphids.

However, some selective insecticides performed unexpectedly. Pirimicarb, often considered selective for aphid control, was highly toxic (>99% mortality) to all parasitoid wasp species, even at rates below the MRFR.

Other insecticides showed variable effects. Sulfoxaflor was highly toxic to D. rapae and A. colemani (93–100%), but less so on A. abdominalis (57–87%). Synthetic pyrethroid toxicity also varied: bifenthrin was lethal to all species (100%), whilst gamma- and lambda-cyhalothrin had minimal impact on A. abdominalis (0–13%) and variable effects on A. colemani (0–30%), but moderate toxicity to D. rapae (~66%).

Among the parasitoids, A. abdominalis appeared the most tolerant, with lower mortality across several insecticides such as gamma-cyhalothrin, lambda-cyhalothrin, and sulfoxaflor, compared to the other two species.

Study 2: Efficiency of aphid control by parasitoids

Across all sites, augmenting parasitoids reduced aphid densities by 40–50% compared to relying solely on natural control (Figure 1). Regionally, this effect was seen in four of five areas, including Victoria, the Riverina, and Southern NSW, but there was no such reduction observed in North and Central NSW.

Figure 1. Marginal means (± standard error) of aphid densities per plant across all augmented and control plots in the spring field surveys.

Insecticide application suppressed biocontrol by resident parasitoids (Figure 2). In augmented and natural control plots, aphid densities were strongly positively correlated with mummification rates, indicating parasitoid populations increased in response to aphid outbreaks. In contrast, no such correlation was observed in insecticide-treated sites, consistent with the toxicity results from study 1, where the insecticides applied by growers to the conventional plots in study 1 were classified as highly or very highly toxic to parasitoids.

Figure 2. Relationship between aphids and mummified aphids. The line shows a linear line of best fit (± 95 confidence interval). Note abundance plotted on a log scale.

Finally, D. rapae dominated across all sites, accounting for 97% of samples, and demonstrated the highest biocontrol potential. In contrast A. ervi and A. colemani struggled to establish populations, comprising only ~1% of samples, despite augmentation efforts.

Growers were asked to share yield and input cost data for each plot. With many sites harvested in November 2024, this data is pending.

Discussion

Both studies offer valuable insights into the potential of combining parasitoid augmentation with selective pesticide use to improve aphid management in grains and horticulture, while also highlighting the complementary synergies between these approaches.

The preliminary findings of Study 2 showed that parasitoid augmentation significantly reduced aphid densities (χ2 = 14.87 d.f=1, p<0.01), particularly in Western Victoria and Southern NSW, with reductions of 40–50%, suggesting augmentation as a viable alternative in suitable regions. Still, the effect of parasitoid augmentation varied significantly across regions (χ2 = 133.12 d.f=6, p<0.01),, with the underlying reasons for this variability are presently being explored.

Further investigations will examine whether spatial and environmental factors drive regional differences, with the aim of using the findings to develop tailored local management recommendations. Future research will also use yield and input cost data when available to explore the economic viability of augmentation compared to conventional control methods

Study 2 illustrated the disruptive effects of broad-spectrum insecticides on parasitoids in a field-based setting, evidenced by lower mummy densities and diminished pest control. This aligns with the laboratory results from Study 1, where broad-spectrum chemicals like methomyl and bifenthrin are shown to cause near-total mortality across all tested parasitoid species. While methomyl and bifenthrin are registered for use in grain crops to manage pests other than aphids, this research highlights their potential off-target effects on beneficial species like aphid parasitoids. Secondary pest incursions—where populations of non-target pest species surge due to the disruption of beneficials—are often a significant consequence of such off-target effects, further complicating pest management efforts.

Study 1 highlights the importance of species- and chemical-specific considerations in pest management. The unexpected toxicity of pirimicarb, traditionally regarded as selective, alongside the variable impacts of sulfoxaflor and synthetic pyrethroids, underscores the need for nuanced recommendations tailored to specific parasitoid-insecticide interactions. Additionally, A. abdominalis demonstrated greater tolerance to insecticides such as gamma-cyhalothrin, lambda-cyhalothrin, and sulfoxaflor, suggesting it may be better suited for use in regions or systems where these chemicals are commonly applied.

Conversely, findings from Study 2 highlight D. rapae as the dominant parasitoid across sites, showing the greatest potential as an augmented biocontrol option. It demonstrated a strong efficacy in suppressing pest populations, particularly in regions with minimal insecticide use.

When chemical control is required, selective insecticides such as flonicamid and afidopyropen show strong potential for integration into IPM strategies. Study 1 indicates their low toxicity to parasitoids, including the dominant species D. rapae, making them compatible with biocontrol efforts Whilst registered in canola, flonicamid and afidopyropen are not yet registered for use in all grain crops, limiting their availability across different cropping systems. Additionally, some growers in Study 2 anecdotally noted upfront costs of selective insecticides compared to broad spectrum as a factor influencing product choice for pest management. However, from a management perspective, these chemicals offer long-term benefits by preserving beneficial insects and contributing to resistance management; thereby prolonging insecticide efficacy and reducing the long-term costs associated with resistance management.

Conclusion

By integrating the findings from two discrete projects, this paper highlights the potential of combining parasitoid augmentation with selective insecticide use to improve aphid management in grain crops. Whilst some results are preliminary and derived from a limited number of chemically treated sites, early findings suggest that parasitoid augmentation could help control aphid populations, particularly in certain regions. The research also emphasises the importance of selecting pesticides that minimise harm to beneficial insects, highlighting the synergies between selective insecticides and biological control strategies for effective aphid management. The combined results of the studies reinforce the importance of a systems-based approach to IPM. Ongoing research will continue to refine our understanding of how these strategies can be effectively integrated into pest control programs.

Acknowledgements

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, the authors would like to thank them for their continued support. The ‘Beneficials chemical toxicity table’ for Australian grains and data generation used in this paper was developed by Rosie Knapp, Robert McDougall, Kathy Overton, Samantha Ward (Cesar Australia) and Ary Hoffmann, Paul Umina (the University of Melbourne), with support from the University of Melbourne and GRDC as part of the Australian Grains Pest Innovation Program (AGPIP) (UOM1906-002RTX). We thank Jacquie Murphy, Lizzy Lowe, Karyn Moore, Lisa Kirkland, Marielle Babineau and Evatt Chirgwin for their assistance. We also acknowledge the South Australian Research and Development Institute, Hort Innovation Australia, CropLife Australia, Biological Services and Bugs for Bugs. Assessing the ecological and economic benefits of controlling aphid pests of canola with parasitoid wasps (CES2307-001RTX) will be delivered by Cesar Australia in collaboration with The University of Melbourne, The Crop Capsules Company, Biological Services, and Bugs for Bugs. This initiative is a GRDC investment and includes in-kind contributions from all project partner organisations.

References

Knapp R, Mata L, Umina P, Miles M, Hoffmann A, Lowe L (2023) Minimising the impact of insecticides on beneficials in broadacre crops. Proceedings GRDC Grains Research Update, Dubbo, February 2023.

Overton K, Ward SE, Hoffmann AA, Umina PA (2023) Lethal impacts of insecticides and miticides on three agriculturally important aphid parasitoids. Biological Control 178, 105143. (10.1016/j.biocontrol.2022.105143)

Contact details

Lilia Jenkins
Cesar Australia
0435 746 668
ljenkins@cesaraustralia.com
info@cesaraustralia.com

Date published
February 2025