Develop and validate a dynamic action threshold tool for aphid management in cereal crops in the Prairies

Project Code: PRR12-040

Project Lead

Chrystel Olivier and Tyler Wist  Agriculture and Agri-Food Canada

Objective

To make available to growers a dynamic action threshold, as part of a spray decision support system for aphid control in wheat and barley crops that incorporates changing numbers of aphids and their natural enemies

Summary of Results

Background

Aphids are periodically a serious threat to wheat, oats, and barley in the Canadian prairies. Several aphid species are known to affect cereals, with the English grain aphid, Sitobion avenae being particularly damaging due to its habit of feeding on wheat heads. Wheat yield can be reduced by 20-30% during an aphid outbreak. Growers currently have limited control options available, with dimethoate and malathion being the only insecticides available for aphid control in cereals. However, these broad spectrum products can also kill natural enemies. Natural parasitoids and predators known to feed on aphids can be abundant in cereal crops and can suppress aphid populations, preventing them from reaching or exceeding the economic threshold. With a sufficient number of natural enemies in the field, chemical sprays may not be necessary.

The current economic threshold triggering a spray action for aphid control in Western Canada is 12-15 aphids per tiller and it does not account for aphid mortality due to natural enemies. Aphids can quickly exceed this threshold if left unchecked and can reduce yield by 35-40% depending on crop stage.

This project aimed to develop a new threshold which would allow growers to account for the presence and suppression by natural enemies when making spray decisions. The goal was to create a tool similar to soybean aphid dynamic action threshold previously developed for soybeans crops in Ontario.

Approaches

About 20 - 40 conventionally managed cereal (wheat, barley and oats) field sites were selected across Saskatchewan and Manitoba to be surveyed during the 2012 and 2013 growing seasons. The species identities and population densities of aphids and their potential predators and parasitoids were determined in these surveyed fields. Aphid mummies (an aphid killed by the larva of a parasitoid wasp and characterized by a swollen brown or black tanned skin attached to a plant substrate) were also counted and the percentage of parasitism was estimated.

Two insect sampling methods, whole plant and sweep netting, were tested for their efficacy to accurately determine insect numbers in the fields. Sweep net and whole plant samples were taken biweekly in 2012 and weekly in 2013. The whole plant method consisted of bagging and removing ten whole plants from each site. Sixty sweep net samples per site and sample date were taken. The number of cereal heads sampled per 60 sweeps through the field was estimated to relate to the number of tillers sampled to assess the economic threshold, currently calculated as number of aphids per tiller. Captured insects in all sites were identified and their mean numbers analyzed for each sampling method.

A mathematical dynamic action threshold (DAT) model was developed which incorporates the total suppressing pressure of naturally occurring predation and parasitism on aphid population growth. The model was based on the previous DAT equation describing the populations of soybean aphids (Aphis glycines) and its natural enemies in soybean crops. Modifications included incorporation of data about the rates of increase in cereal aphid populations and consumption by natural enemies. The voracity levels, expressed as the number of aphids killed by predators and parasitism in 24 hours, were found in the literature and incorporated into the DAT model as the Natural Enemy Unit (1 NEU = the number of each natural enemy species required to kill 100 aphids in 24 hour). The performance of the model was validated in three fields where aphids and natural enemies were well established.

Results

English grain aphid (S. avenae) was the main species of cereal aphids present during the two years of this study. Two other aphid species known to attack cereal crops, the greenbug (Schizaphis graminum) and the Oat-birdcherry aphid (Rhopalosiphum padi) were also present, but in lower numbers. The most numerous aphid predators recorded were ladybugs (Coccinella septempunctata and Hippodamia tredecimpunctata) and green lacewings (Chrysoperla carnea and Chrysopa occulata). Minute pirate bugs (Orius tristicolor), and damsel bugs (Nabis americoferus) (in Manitoba) were also present in lower numbers. Aphid parasitism rates were low across the surveyed sites with adults of Aphidius avenaphis, A. colemanni, Aphelinus varipes, A. asychis and A. alpibodus either collected via sweeping or reared from aphid mummies. Overall, natural enemies contributed to suppressing S. avenae populations in cereals.

Sweep net sampling for aphids and their natural enemies proved superior and captured more of every insect species than the whole plant sampling method. Whole plant sampling, however, was useful for detection of aphid mummies and for quantifying the number of aphids per individual plant. It was also calculated that current economic threshold (12-15 aphids per tiller) equals to 29,033 aphids per 20 sweeps.

The new DAT model proposed for cereal aphids is designed to estimate the effects of all natural enemies present to predict their impact on the growth of aphid populations in cereals in one week. For instance, the DAT model predicted that 2,687 aphids per 20 sweeps, if uncontrolled by predators or parasitoids, will exceed the economic action threshold in seven days. Validation trials indicated that the model predictions were similar to the actual growth of the aphid population over the entire growth season. However, the model seems to overestimate the pressure and the rate of aphid kills by natural enemies at the early stages of field infestations by aphids, predicting the collapse of aphid populations before these actually occur in the field.

Typically, insecticide applications are recommended at the action threshold when aphid populations are predicted to exceed the economic threshold. The DAT developed in this project improves upon the existing action threshold through incorporating the potential for aphid control by natural enemies by predicting their effect on aphid population growth. As such, this DAT recommends the application of pesticides only when the population of predators and parasitoids are insufficient to prevent populations of cereal aphids from reaching the economic threshold.

This project demonstrated that incorporating biological control as an element of integrated aphid management approach in cereals crops is possible. More research is needed however, to further refine the DAT model into a fully operational and viable tool to help growers in making informed spray decisions. It is anticipated that the use of this tool will help to prevent unnecessary pesticide applications when unwarranted, thus protecting the beneficial insects which are important in aphid control.

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