Early season rosy apple aphid control
Summary
Hand-removing rosy apple aphids by pinching off colonized leaves early in the season (from tight cluster to bloom) reduces overall rosy apple aphid abundance in the orchard. This practice complements the biological control provided by predators, thereby reducing overall rosy apple aphid damage and the need to apply summer oils.
The problem
Rosy apple aphids are a common pest in apple orchards. Their saliva from feeding on apple foliage causes leaves to pucker and fruitlets to become runted and deformed, hence unmarketable. Oils approved for use in organic agriculture can be sprayed on the trees at specific times in the season to provide some control, but in organic farming there is no good “reactive” solution for controlling rosy apple aphids if their populations are allowed to get large. Rosy apple aphids reproduce exponentially, so early-season control is the only effective way to manage this pest in an organic system.
In organic farming, we rely heavily on predators for controlling rosy apple aphids. Some examples of highly effective early-season predators include parasitoids, ladybug larvae and adults, and lacewing larvae. Unfortunately, predator populations do not always show up in time, or in significant enough numbers, to prevent early-season damage from rosy apple aphids. For instance, weather can sometimes delay the exponential increase of particular predator populations, while leaving the aphid population relatively unaffected. By the time predators do show up in force, it can sometimes be too late to keep damage from rosy apple aphids below an economic threshold.
Before taking any pest control action, we monitor to see if a given pest is abundant enough to justify taking control measures. In high density plantings in organic orchards, locating one or two bud clusters per tree with rosy apple aphids early in the season is cause enough for taking action (Edwards 1998: 46).
Hand-removal as a control method
To prevent aphid populations from getting out of hand, we have experimented with early-season removal by hand of leaves on which rosy apple aphids are found. We hunt for rosy apple aphids beginning at tight cluster, when their numbers are very low (typically only one or two aphids per colonized bud cluster). When we find the aphids, we pinch off the leaf the aphids have colonized and let it fall to the orchard floor. We don’t crush the aphids we find, since we still want predators to sense their presence and find them. While carrying out hand-removal, we also monitor for predator abundance by inspecting some percentage of aphid-colonized clusters. Another reason for not crushing the aphids we find is that we do not want to inadvertently crush predators or their eggs.
Importantly, our goal is not to eliminate rosy apple aphids from the orchard, since doing so would act against our goal of maintaining predator populations. Our goal is to keep rosy apple aphid damage to a level that is economically insignificant. Since every year we hand-thin our apple trees to remove excess crop anyway, a low level of rosy apple aphid damage can easily be tolerated.
We have found that in high-density apple trees, the vast majority of early-season rosy apple aphids tend to colonize bud clusters in the bottom two thirds of the tree. This means that we can do a good enough job without using ladders. On standard-sized trees, rosy apple aphids tend to colonize the inner parts of the tree first, and ladders are necessary for reaching them.
Results
While we have been careful and systematic in our observations of rosy apple aphid populations, we have not attempted to translate these observations into precise numerical quantities. Our results are reported in general terms below.
In 2018, we had a bad year for rosy apple aphids. It was the first spring after we began transitioning our farm from conventional to organic, and therefore the first spring since we had stopped spraying conventional pesticides. Our farm ecosystem had not had enough time to build up predator abundance and diversity to levels capable of controlling aphids. We experienced more apple loss to aphid damage than probably ever before, and some trees were even sickened to the point that their yields went down the following season. The upside of this experience is that we learned what it looks like to have a bad year for rosy apple aphids.
In 2019, starting at tight cluster, Katie hand-removed aphid colonies reachable from the ground on all apple trees located along all the edges of high-density Ambrosia and Pink Lady blocks. Several known hot spots in our low-density Spartans were also targeted for hand-removal using ladders. Monitoring these areas throughout the summer, we observed that the damage from rosy apple aphid was minimal on the edges of rows compared to the damage seen in the previous year. Hand-removal of aphids along the edges of Gala blocks was also carried out, though not until petal fall, when aphid populations had already substantially multiplied. We observed that aphid damage was worse in Gala blocks than elsewhere. From this, we deduced that hand-removal should be completed before bloom.
Multiple generations of rosy apple aphid on Gala leaf after petal fall.
In 2020, hand-removal was extended to all trees within every second row in high density blocks, in addition to the areas targeted for hand-removal in 2019. All hand-removal was carried out before bloom. Observations throughout the season were that overall damage from rosy apple aphids was minimal.
In spring 2021, we could not find enough rosy apple aphids while monitoring at tight cluster to make hand-removal worthwhile. It is possible that early-season drought had delayed or prevented the emergence of many aphids. The time normally spent hand-removing aphids was spent instead clipper-removing bud clusters infected with powdery mildew (which we found to be more common than usual in spring 2021). Rosy apple aphid populations did expand later in the season, and while overall apple damage was still minimal, there were some new and unanticipated hot spots (mainly in our Spartans) where aphid damage was moderate.
Discussion
Our experience suggests that early-season hand-removal of rosy apple aphid clusters is an effective practice that complements our reliance on biological control. We have also found that for hand-removal to be effective, it must be carried out before bloom. Together, hand-removal and biological control reduce the need for summer oil sprays to control rosy apple aphids. This benefits our ecosystem, as well as our farm’s bottom line.
We should note that reduced damage from rosy apple aphids has coincided with a gradual increase in predator populations and biodiversity on our orchard. This means that there is uncertainty about the degree to which our success in controlling aphids has been due to hand-removal, rather than simply due to increased biological control. In the future, we intend to continue hand-removal in years when monitoring suggests a need for it, and as long as we have time in our management schedule to spare.
One of the many benefits of being a family-scale farm is that we do have the time to engage in practices like hand-removing aphids. (In fact, it is one of Katie’s favourite orchard jobs). On larger farms, accessing sufficient labour to carry out time-sensitive practices like hand-removal can be difficult. This is one reason why larger farms have to rely more on chemical pesticides than family-scale farms like ours.
Observations
Leaf removal and fruitlet development
One question that the method of hand-removal raises is whether the act of removing one or more leaves from a bud cluster could have an effect on the developing fruitlet in that cluster, potentially causing it to develop poorly (e.g. resulting in small-sized fruit).
When we posed this question to Dr. Hao Xu at the Summerland Research Station, she provided us a very informative response, which is partially reproduced here:
Usually in the early spring, the photosynthates that the young, still-expanding leaves assimilate are mainly used to support their own growth. The growth of these young leaves, and the early stages of fruitlet set, also partially rely on the reserves in the twigs and branches. In these early days, the young leaves and fruitlets are thought to be “competitors” for carbohydrate; the loss in the competition leads to June drop. However, once the leaves grow into fully expanded and mature leaves, their function switches from carbohydrate sink to source, and they start to provide carbohydrates to the adjacent fruitlets. The leaves on the fruit-bearing twigs are thought to be the main sugar providers to the fruits. They tend to have higher photosynthetic capacity, compared to the leaves on the non fruit-bearing twigs. This is partly because the enzyme reaction of photosynthesis is accelerated when its products are quickly removed into the strong sink of developing, sugar-demanding fruits nearby. On the hot summer days, the leaves transpire water and cause localized cooling, which helps to prevent sunburn. So, if you can keep the leaves on fruit-bearing twigs, that would be beneficial for the overall dry matter accumulation, water status and quality of the fruits. A rough estimation would be at least 150 cm2 leaf area for one fruit, which are 4-5 mature and healthy leaves on fruit-bearing twigs, with good light interception. (Dr. Hao Xu, personal communication, 2020).
Dr. Xu then goes on to explain that pinching off younger leaves would at least be more economic than pinching off mature leaves, from a tree’s perspective, and that carbohydrates do end up getting circulated throughout the tree in complex ways. We conclude that there could indeed be an effect on the development of a fruitlet if its nearby leaves are removed early in the season, though it is also possible that this effect could be compensated for.
Does this mean that removing aphid-colonized leaves from bud clusters might do more harm than good? We don’t believe so. After all, if aphid populations are allowed to get too large, their feeding can sicken a tree to the point of reducing yields in the following year. Hand-removal is warranted whenever it prevents a population explosion that leads to this dire outcome.
In the future, we may attempt to mark bud clusters which have had one or more of their leaves removed, to see whether the growth of apples on these clusters is affected.
Observations on aphid migration
Rosy apple aphids almost always migrate out of the apple trees midsummer and spend several months on secondary hosts on the orchard floor. One of the main secondary hosts on our site is broad-leafed plantain (Plantago major). Interestingly, we have not noticed a higher abundance of rosy apple aphids on trees in areas of the orchard where plantain is more abundant. In fact, the spatial distribution of these populations on our orchard tends to pattern along the following lines:
Plaintain tends to be more abundant in and around compacted soil and in shadier areas of the orchard.
Rosy apple aphids tend to be more abundant along tree row edges and in warm areas exposed to the southern sun.
This joint distribution pattern suggests that rosy apple aphids are selective in how they choose sites to lay their eggs, and are willing to migrate some distance from their secondary hosts within the orchard to access those sites. In particular, female aphids seem to migrate in order to lay their eggs where temperature units accumulate the fastest in the spring, thereby ensuring their progeny have an early start the following season. Smart mommas!
By Katie Sardinha
Look closely: it’s an aphid giving live birth!
References
Edwards, Linda. 1998. Organic Tree Fruit Management. Keremeos, BC: Certified Organic Associations of British Columbia.
