IntroductionCereal aphids can vector Luteoviridae viruses commonly referred to as Yellow Dwarf Viruses (YDV). Rhopalosiphum padi L is one of the more common cereal aphid vectors of YDV found in Canterbury. Foliar insecticides offer a method for reducing the aphid vector populations and hence the risk of YDV incidence. One of the principal factors in developing a strategy for aphicide inputs is knowledge of the persistence of products being used. The impact of the aphicide on the natural enemies of aphids is also important as they contribute to the control of aphid populations. Work in Australia in cotton shows that Karate Zeon® has a severe impact, Transform™ a low impact, and Pirimor® has a very low impact on these predators. Mavrik® was not included in the publication but other work gives conflicting ratings on its impact on aphid predators, from low harm to harmful. This information does not always relate directly to predators in New Zealand as they are not necessarily the same species but gives a general guide. To reduce the risk of resistance growers should rotate with insecticides from a different chemical group (see Table 1). The aim of this field trial was to evaluate the persistence of selected insecticides (Karate Zeon®, Mavrik®, Pirimor®, Transform™, Eco-oil®, and Poncho®-treated seed) to a cereal aphid species, R. padi L., to provide growers with information on their residual activity against cereal aphids.

Field trialThe barley cultivar ‘Sanette’ treated with a seed treatment of either the fungicide Raxil® or Raxil® + Poncho® were sown on 8-9 September 2016 into individual planter bags. The plants were kept in an insect-proof shade house. At GS 13-14 on 10 October 2016, all plants were transferred to the field and sprayed with the insecticide treatments or the water control (Table 1). When the foliage had dried, four plants from each treatment were transferred to the entomology laboratory at Plant & Food Research. Ten large wingless aphids (R. padi) were attached to a leaf using clip cages (Figure 1). After 2 days, the number of live aphids was recorded. Every 2 days thereafter, four plants from each treatment were transferred from the field to the laboratory and 10 large wingless aphids were applied to each plant using the clip cages, until the insecticide-treated plants of each treatment had a similar number of live aphids as compared to water-treated plants, or when a mean of ≥ 50% live aphids was recorded. The final assessment was on 28 October 2016. Insecticide persistence (residual insecticide activity) was defined as the number of days that 30% or fewer live aphids remained on a plant. The active ingredients and rates tested are listed in Table 1.

Key points

• A foliar aphicide persistence trial was set up in spring sown barley in September 2016 at Plant & Food Research, Lincoln.

• The trial tested aphicides from four modes of action: synthetic pyrethroids (Karate Zeon® and Mavrik®), carbamates (Pirimor®), sulfoxamines (Transform™) and chloronicotinyl (the seed treatment Poncho®). It also tested Eco-oil®.

• The seed treatment Poncho® was shown to be the most persistent product of those tested, giving 13.4 days of persistence from the date the foliar insecticide treatments were applied and 43 days after sowing.

• Transform™ at a full rate gave 10 days persistence which was significantly longer than all other treatments except for Karate Zeon® 40 ml/ha at 6.5 days.

• Karate Zeon® at a half rate, Mavrik®, Pirimor® and Eco-oil® gave a similar persistence of 3.8 to 5.2 days.

• In addition to persistence, other factors need to be considered such as price, rotating modes of action for anti-resistance and choosing insecticides more compatible with beneficial insects.

Arable UpdateCereals: Issue 214

September 2017

Foliar aphicide persistence on spring barley

Table 1. The treatments used in the insecticide residue trial; foliar insecticides applied based on a water rate of 200 l/ha. Note: 25 ml/100 l of a non-ionic wetting agent (Contact™ Xcel) was added to each insecticide before applying to plants.

Insecticide Active ingredient IRAC insecticide group Rate (ml/ha)Water (control) * * *Karate Zeon® 250 g/l lambda-cyhalothrin Synthetic pyrethroid (Group 3) 40Karate Zeon® 250 g/l lambda-cyhalothrin Synthetic pyrethroid (Group 3) 20Pirimor® 500 g/kg pirimicarb Carbamate (Group 1A) 200 (g/ha)Mavrik® 240 g/l tau-fluvalinate Synthetic pyrethroid (Group 3) 150Seed treatment (Poncho®) 600 g/l clothianidin Chloronicotinyl (Group 4A) 60 ml/100 kg seedEco-oil®1 851.5 g/l canola oil 10 ml/lTransform™ 240 g/l sulfoxaflor Sulphoxamines (Group 4C) 100Transform™ 240 g/l sulfoxaflor Sulphoxamines (Group 4C) 50

1 Eco-oil® label rate range 5 – 10 ml/l

Figure 1. ‘Sanette’ barley plants with a small clip cage containing 10 large wingless Rhopalosiphum padi cereal aphids attached to each plant.

ResultsPlants grown from Poncho®-treated seed kept numbers at or below 30% live aphids per plant for the longest out of all insecticides tested (mean = 13.4 days after the foliar insecticides were applied to the other treatments and 43 days after sowing), followed by the full- and half-rates of Transform™ (mean = 10 and 7.9 days respectively after spraying) (Table 2, Figure 2). The applications of Karate Zeon® (full and half rate), Mavrik®, Pirimor® and Eco-Oil® had a residual activity against the aphids of 3.8 - 6.5 days (Table 2, Figure 2). There was a moderate amount of variability in the trial reflected in a high cv% and LSD.

The method used to expose aphids to insecticide residues in the present study differed from that used in previous studies, to eliminate the impact of cold, windy and/or wet weather on aphid survival. In previous studies, plants were sown in the field. Aphids were added to the same plants over the course of the trial and these were left in the field.

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Table 2. The residual activity of the insecticides was estimated as the number of days after spraying, where ≤30% of aphids had survived after 48 hours on the plants. Analysis: A polynomial regression model was fitted to each plot and the days after spraying that the line stayed below 30% aphids alive was analysed by ANOVA (analysis by FAR).

Treatment ≤30% aphids per plant, (days after spraying)Karate Zeon® full rate 6.5Karate Zeon® half rate 3.8Pirimor® 5.2Mavrik® 4.7Poncho® seed treatment 13.4 (43 days after sowing)Eco-oil® 4.5Transform™ full rate 10.0Transform™ half rate 7.9Mean 6.2LSD 3.9cv% 43

Figure 2. The mean percentage of live aphids per plant (95% confidence intervals). The green line indicates the residual activity threshold of 30% live aphids. The red line is at 50%, used as the threshold to show the insecticide residues are no longer active in reducing numbers of live aphids. Kf = Karate Zeon® full rate, Kh – Karate half rate, P = Pirimor®, M = Mavrik®, S = Poncho®-treated seed, E = Eco-oil®, Tf = Transform™ full rate, Th = Transform™ half rate.

The method described here kept the plants in the field after spraying so that insecticides were subjected to the weather. It then confined aphids onto plants in the laboratory, thus eliminating the effect of weather on aphid survival.

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Assessment Date 12/10 14/10 16/10 18/10 20/10 22/10 24/10 26/10 28/10 30/10

Assessment Date 12/10 14/10 16/10 18/10 20/10 22/10 24/10 26/10 28/10 30/10

Assessment Date 12/10 14/10 16/10 18/10 20/10 22/10 24/10 26/10 28/10 30/10

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ConclusionsA previous FAR funded project undertaken by Plant & Food Research in 2002 using the method where aphids were applied in the field to spring-sown ‘Optic’ barley plants gave similar results for Karate Zeon®. The residual activity for full-rate Karate Zeon® was 4.3 days, and half-rate Karate Zeon® 2.7 days. Pirimor®-treated plants had a shorter residual activity of less than one day (FAR Cereal Update C126).

In spring sown cereals the difference in persistence between the foliar applied insecticides is about six days with Transform™ having the longest persistence. Previous trial work has shown that full rates of Karate Zeon® (25 days), and Mavrik® (15 days) are more persistent when applied in winter (FAR Cereal Update 205). Over winter 2017 the trial was repeated on autumn sown wheat with the new methodology of confining aphids onto plants in the laboratory. In contrast to the earlier trials the persistence of the synthetic pyrethroids was much reduced. This trial needs to be fully analysed and will be reported fully in a future update.

Persistence is one factor in selecting an aphicide. Other factors to consider include price, rotating modes of action for anti-resistance and choosing insecticides more compatible with beneficial insects.

AcknowledgementsThis work was carried out by staff from Plant & Food Research, Lincoln as part of the Sustainable Farming Fund project ‘Managing BYDV in Cereals Sustainably’ (project number 404939). Funding assistance was provided from MPI SFF, Bayer CropScience, Dow AgroSciences, Luisetti Seeds and PGG Wrightson Grain.