Assessments of D. suzukii in fruit
There were linear relationships between larvae flotation (f) and adult D. suzukii emergence (e) measurements determined on equivalent numbers of cherries (f = 0.298e + 0.163, r = 0.666, P < 0.001) and raspberries (f = 0.192e + 0.549, r = 0.686, P < 0.001) taken from plots in assessment weeks 2, 3 and 4 in each of the experiments. This indicated that the larvae flotation method detected 29.8% and 19.2% of the potential adult D. suzukii emergence from the cherry and raspberry fruit samples.
D. suzukii control in cherry, 2021
The first D. suzukii adults emerged 5–8 days after incubation, with the majority emerging by day 13. Similar proportions of female and male D. suzukii emerged in all boxes; overall, the ratio of D. suzukii females to males was 54:46.
There were significant effects of spray treatment (Χ26 = 56.26; p < 0.001) and assessment week (Χ22 = 66.68; p < 0.001) on the numbers of D. suzukii adults that emerged from cherry fruit. Approximately three times more D. suzukii adults emerged from the untreated control fruit in week 4 than in weeks 2 and 3 (Fig. 1) (z > 4.47; p < 0.001). However, by week 4, most fruit was over-ripe, and commercially would have been picked in week 3.
The low rate insecticide sprays without baits reduced D. suzukii adult emergence from cherry fruit, although the effect was only significant in week 4 (z = 3.62; p < 0.001) (Fig. 1). Overall and in individual assessment weeks 2 and 3, the full field rate, reduced rate and low rate with molasses bait spray applications resulted in further reductions in D. suzukii (z ≥ 3.20; p ≤ 0.011). However, only the full and reduced rate treatments significantly lowered adult emergence compared with the low rate without bait treatment in week 4 (z ≥ 2.68; p ≤ 0.007). There were no significant differences in adult emergence between the full and reduced insecticide rates, or between the low rate insecticide treatment with molasses or Combi-protec baits.
Fruit sampled three days before the treatments were applied contained a mean 1.5 (± SE 1.0) larvae per fruit before the introduction of laboratory reared D. suzukii. There were significant effects of spray treatment (Χ26 = 45.34; p < 0.001), assessment week (Χ23 = 42.28; p < 0.001), canopy section (Χ22 = 37.78; p < 0.001) and of treatment × assessment week (Χ218 = 47.30; p < 0.001) and treatment × canopy section (Χ212 = 40.24; p < 0.001) interactions on the numbers of larvae in cherry flotation tests.
There was a large increase in the number of larvae detected in week 4 compared with earlier assessment weeks in the untreated control plots (Fig. 2) (t108 = 3.70; p = 0.001). In assessment week 1, the reduced rate of spinosad resulted in a significant reduction in the number of larvae detected in flotation tests (z = 2.62; p = 0.046); differences between other treatments were not significant (Fig. 2). Overall, the low rate insecticide without baits reduced the number of larvae in fruit compared with the untreated control (t108 = 3.26; p = 0.007), although the effect was only significant in week 4 (t108 = 3.88; p < 0.001) (Fig. 2). In weeks 2, 3 and 4, the full field rate and reduced rate sprays resulted in significantly fewer larvae than the low rate sprays without baits (t108 > 2.67; p ≤ 0.041) (Fig. 2). Averaged across all weeks, the full rate, reduced rate and low rate with bait sprays were not significantly different and all resulted in fewer larvae than the low rate without bait sprays (t108 > 3.41; p ≤ 0.004) (Fig. 2). This difference between the low rate sprays without bait and other insecticide treatments was only significant for samples taken from the mid and far distances from the spray nozzle on the tree canopy (t108 > 2.98; p ≤ 0.017) (Fig. 2).
Although the numbers of larvae in untreated control plots were highest in the mid tree canopy section (t108 = 2.35; p = 0.05), equally good control of D. suzukii was obtained with full and reduced insecticide rates and low rate insecticide with bait spray treatments in all tree canopy sections. However, for the low rate insecticide without bait spray, larvae numbers were significantly higher in the mid and far canopy sections than in fruit collected from the sides of the trees closest to the sprayer nozzle (t108 ≥ 2.68; p ≤ .021) (Fig. 2).
D. suzukii control in raspberry, 2020
Fruit sampled three days before the treatments were applied contained a mean 0.2 (± SE 0.1) larvae per fruit before the introduction of laboratory reared D. suzukii. There were significant effects of spray treatment (Χ24 = 37.38; p < 0.001), assessment week (Χ26 = 133.90; p < 0.001), canopy section (Χ22 = 9.90; p = 0.007) and of treatment × assessment week (Χ219 = 42.04; p < 0.001) interaction on the numbers of larvae in flotation tests.
During weeks 1, 2 and 3, D. suzukii larvae numbers in samples from untreated control plots were low but increased from week 3 onwards (Fig. 3). Overall, the low insecticide rate without bait reduced larvae numbers (z = 2.74; p = 0.022) although the effect was not significant in individual assessment weeks. Insecticides at full field rate or low rate with bait almost completely controlled D. suzukii larvae numbers and remained significantly lower than untreated controls until week 5, two weeks after the final insecticide sprays were applied (z > 3.15; p < 0.003) (Fig. 3). Thereafter, D. suzukii larvae numbers in low rate insecticide with bait spray treatments increased at a similar rate to those in untreated plots or plots treated with low rate insecticides alone. By week 9, six weeks after the final insecticide sprays were applied, larvae numbers in the full rate treatment remained significantly lower (z > 3.28; p < 0.008) than in the untreated control and low rate without bait treatments.
During weeks 1 to 5, there was no significant effect of the height at which fruit samples were taken from plants on the number of larvae detected in fruit samples in flotation tests. In weeks 7 and 9, samples taken from the top of plants (above 1.0 m) had significantly more larvae than samples taken from the bottom (below 0.6 m) of plants (z ≥ 2.99; p ≤ 0.008). However, compared with treatment effects, there were only small differences in the number of larvae detected in fruit samples taken from different plant heights, and the effect of plant height within individual treatments was not significant (data not shown).
Insecticide residues in cherry fruits
Insecticide residue concentrations detected in cherries picked immediately after spraying were within the EU MRLs for spinosad (0.2 mg kg− 1 fruit), cyantraniliprole (6 mg kg− 1 fruit) and acetamiprid (1.5 mg kg− 1 fruit). Residue levels of acetamiprid were < 0.02 mg kg− 1 fruit in all cherry fruit samples.
There was no significant difference between residue concentrations of fruit samples taken from the same treatments after the second spinosad (spray 3) or cyantraniliprole (spray 4) applications; means of these values are therefore shown in Table 3. There were significant effects of treatment (F4,15 > 8.22; p < 0.001), tree canopy section (F2,15 > 27.84; p < 0.001) and treatment × canopy section interaction (F2,15 > 6.07; p ≤ 0.001) on the residues of spinosad and cyantraniliprole detected in cherries. Where insecticide residues were detected, these were significantly higher in fruit samples taken from the sprayed sides than from the mid or far sides of the trees (t15 > 13.56; p < 0.001). No spinosad or cyantraniliprole residues were found in fruit taken from the far sides of trees or from any tree canopy sections sprayed with the low rate insecticide with bait. The higher residues of spinosad in samples taken from full rate plots than from reduced rate plots (t15 > 5.39; p < 0.001) corresponded with the higher spinosad concentration in the full rate applications.
Spray application and deposition analysis
Full and reduced rate insecticide applications took 97 seconds per compartment compared with 10 seconds for low rate insecticide sprays with and without baits. Measured spray volumes per tree were 339 (± 13) ml for the full and reduced rates and 26 (± 3) ml for the low rate sprays, corresponding with 102% and 101% of the target values in Table 1. Total amounts of spinosad applied in two full, reduced or low rate applications were 165, 64 and 6 mg tree− 1 respectively. Total amounts of cyantraniliprole applied in two full or low rate applications were 123 and 5 mg tree− 1 respectively.
The difference in spray deposition coverage between the low rate with Combi-protec applications at 40 and 80 L ha− 1 were not significantly different; only the former results are therefore presented here. There were significant effects of spray treatment (F2,1008 = 430.89; p < 0.001), canopy section (F2,1008 = 316.82; p < 0.001) and height (F2,1008 = 5.79; p = 0.003), leaf surface (F1,1008 = 9.47; p < 0.001) and interactions between these factors (F8,1008 ≥ 3.07; p < 0.01), except canopy section × leaf side, on spray deposition coverage. For the full rate spray (500 L ha− 1), leaf surfaces nearest the spray nozzle received the highest coverage (> 20%) and, with the exception of upper leaves at the top of trees, leaves furthest from the nozzle received the lowest spray deposition (≤ 0.5%) (t1008 > 81.60; p < 0.001) (Fig. 4).
For the bait spray (40 L ha− 1), leaf surfaces nearest the spray nozzle received a higher coverage than leaves in the middle of the canopy or furthest from the spray nozzle (t1008 > 32.22; p < 0.001). Spray deposition coverage was significantly (t1008 > 2.73; p < 0.018) higher (between 5 and 134 times higher in corresponding positions) for the full rate application than for the low rate with Combi-protec application, except on leaves furthest from the spray nozzle at the middle tree height (both leaf surfaces) and top of trees (upper leaf surface) (Fig. 4).
Spray deposition data and analysis of full field rate and low rate with bait spray applications for raspberry are presented in Noble et al. (2021).
No phytotoxicity symptoms were observed on the foliage of any of the cherry trees or raspberry plants.