LAUSR

Simple Summary Commercial orchards are amongst the most intensively sprayed crops, and alternative methods have to be found to replace pesticides. Limiting water and nitrogen (N) supply has shown to be effective in reducing aphid infestations under controlled conditions. To evaluate how far these techniques could be transferred to orchards subject to production constraints, an experiment was performed in a commercial orchard planted with two varieties differing in precocity and vigour. Limiting supplies of both water and N to trees was shown to reduce the severity of aphid infestation (green peach aphid, mealy plum aphid, and leaf curl aphid), although reducing only water supply was less effective. At shoot level, the composition and development of the infested shoots were only slightly affected by treatment, thereby indicating that aphids colonize shoots of similar condition, whose numbers are modulated by nutrition treatments. These results were consistent with variety and year. Limiting water and N supplies contributes not only to the control of aphid infestations, but also reduces nitrate leaching and the use of water, the consumption of which will inevitably need to a decrease due to climate change. However, the efficiency of aphid control could be enhanced by complementing these practices by other techniques such as adapted pruning or changes to ground cover. Abstract Peach orchards are intensively sprayed crops, and alternative methods must be found to replace pesticides. We intend here to evaluate if limiting water and nitrogen (N) supply could be effective in controlling aphid infestation in commercial orchards. N and water supply were therefore either unrestricted or restricted by 30% only for water, or for both water and N, in 2018 and 2019 on trees of two contrasting varieties. Natural infestations (green peach aphid, mealy plum aphid, leaf curl aphid) were monitored regularly at tree and shoot level. Infested and control shoots were compared for their development during the infestation period, their apex concentrations of total N, amino acids, non-structural carbohydrates, and polyphenols at infestation peak. At tree level, limiting both water and N supplies decreased the proportion of infested shoots by 30%, and the number of trees hosting the most harmful specie by 20 to 50%. Limiting only N supplies had almost no effect on infestation severity. At shoot level, the apex N concentration of infested shoots was stable (around 3.2% dry weight) and was found to be independent of treatment, variety, and year. The remaining biochemical variables were not affected by infestation status but by variety and year. Shoot development was only slightly affected by treatment. Aphids colonized the most vigorous shoots, being those with longer apical ramifications in 2018 and higher growth rates in 2019, in comparison with the controls. The differences were, respectively, 40 and 55%. It was concluded that a double restriction in water and N could limit, but not control, aphid infestations in commercial orchards.