The impact of insecticides and miticides on beneficial arthropods in Australian grains

Take home messages

• A guide has been developed outlining the non-target impacts of insecticides and miticides commonly used in grains on natural enemies of grain crop pests.
• There is great diversity in the impact that these chemicals can have on natural enemies.
• This research has confirmed that many active ingredients considered to be ‘soft’ or ‘selective’ have low impacts on beneficials, even under laboratory conditions.
• Our testing revealed that some species demonstrated unexpected tolerances towards active ingredients typically considered to be broad-spectrum products (for example gamma-cyhalothrin).
• Consulting this guide will allow growers to make more informed choices about which active ingredients to use when controlling pests within their crops while preserving natural enemies.

Background

Insecticides and miticides are the primary tool by which Australian grain growers control pests within their crops, with a focus on prophylactic use of broad-spectrum chemicals such as organophosphates and synthetic pyrethroids (Umina et al. 2019). While these are generally effective at preventing yield losses, a downside of the widespread use of these chemicals is the potential for non-target effects (Overton et al.2021). Some of the organisms that can be inadvertently impacted by pesticides include those which can themselves act as agents of pest control, primarily arthropods that serve as predators and parasitoids of pest organisms (Overton et al.2021). This can lead to situations where the application of insecticides results in secondary pest outbreaks due to killing off organisms that were providing biological pest control services, with these beneficial organisms and the services they provide often not noticed until they are lost (Naranjo et al.2015).

While there is a growing awareness of the benefits that these biological control agents can play as part of Integrated Pest Management (IPM) programs, uncertainty surrounding the impact of pesticides on these agents can make IPM programs difficult to implement. To help tackle this issue, guides have been produced for both the cotton (CRDC and Cottoninfo 2021) and horticultural industries (Hort Innovation 2020) in Australia which outline how commonly used insecticides and miticides can affect biological control agents. This has allowed growers and agronomists to make more informed decisions about what chemicals to use in pest control, in particular promoting the use of chemistries that can provide sufficient pest control without compromising locally abundant predators and parasitoids. While these two guides represent a valuable data source, industry differences in the chemicals used, maximum registered field rates, growing environments and key pests mean that a guide specific to the grains industry is required to provide the best possible guidance to growers and advisors. This paper outlines the process of producing this guide and presents our initial findings. A more detailed version of the guide will be available online from early 2022 and updated regularly as more data is collected.

Methods

To determine the impact of insecticides and miticides on natural enemies, we exposed organisms representing key beneficial groups to chemicals in a series of standardised laboratory assays, following protocols developed by the International Organisation for Biological Control (Hassan et al. 1985). Using a Potter Tower, we sprayed Petri dishes with selected chemicals at a rate per cm2 proportional to the labelled application rate per hectare. Chemicals used were selected based on consultation with growers and chemical industry representatives regarding what active ingredients are most commonly used to control invertebrate pests in Australian grains. Chemicals were generally tested at 100% and 10% of the maximum registered field rate (MRFR) within Australian grain crops according to the APVMA (APVMA 2021), unless application rates less than 10% of MRFR are commonly used, in which case this lower rate was tested along with the 100% rate.

After the spray deposits dried (30-60 minutes), individuals from arthropod species representative of key beneficial groups were placed in Petri dishes and mortality was monitored over the next 48-72 hours, depending on species. In accordance with IOBC protocols, we used vulnerable lifecycle stages when conducting testing, with juveniles used in assays on predatory species and adult stages used for testing of parasitoids. The chemicals and arthropods used are shown in Table 1.

Each time an assay was conducted, in addition to the chemicals of interest, we also tested mortality rate of organisms exposed to a “negative” control (water) and a “positive” control – a highly toxic industry standard consisting of either an organophosphate (chlorpyrifos or dimethoate) or a synthetic pyrethroid which was expected to be particularly toxic. Gamma-cyhalothrin was initially used as the representative synthetic pyrethroid, however, a number of assays showed lower than expected mortality rates of organisms exposed to this chemical (see results), so we later switched to bifenthrin.

Arthropods were primarily obtained from the commercial suppliers Biological Services^TM and Bugs for Bugs^TM. However, as hoverflies, spiders and snout mites are not commercially available, we used a combination of field collections and laboratory rearing of these groups to obtain individuals of appropriate life stages for assays.

Results and discussion

A simplified summary of the findings from our assays, along with data from comparable studies from the academic and grey literature, is shown in Table 1. This work is ongoing, and this table should be considered a snapshot of the data available at the time this paper was prepared (Jan 2022) rather than a definitive final document.

Table 1: Impact of insecticides and miticides on beneficial arthropods in Australian grains. Note that some rankings cover several categories of toxicity due to variation amongst species in a group or between studies. Toxicity ratings are based on IOBC protocols, with a rating of L representing <30% mortality, M 30-80%, H 80-99% and VH >99% mortality. Chemicals are listed in descending order from least to greatest overall toxicity when averaged across all organisms tested.

1 – Pirimicarb has been included twice due to large variations that exist in the MRFR for its use in different crops.

2 – While Sulfoxaflor is registered at rates of up to 100 g/Ha a.i. for Greenhouse Whitefly, this is an infrequent pest so we have focused here on the more industry relevant rate of 50 g/Ha a.i.

3 – While Thiodicarb is registered at higher rates for the control of Helicoverpa in maize, such control is often not economical, and thus we have focused here on the more industry relevant rate used in pulse crops.