Prune aphids (mealy plum and leaf curl) are the key insect pest in prune production. High populations of these aphids can reduce tree vigor and fruit sugar content. Low fruit sugar = higher fruit dry away = less money for growers.
There are two basic strategies for aphid control. The first strategy is to spray before bloom – carefully, every row – to guarantee you won’t have a problem in the spring. The second strategy is to wait and watch and be prepared to spray if aphids show up. Because we do not have an effective aphid monitoring program, it is not possible to monitor and effectively predict if you will have spring aphids.
The prebloom program involves investing in a spray before you know for sure that aphids will be a problem. Usually, orchards with a history of aphids will have spring aphid populations. The good news is that a prebloom spray, if properly applied, is very cost effective. If you wait until the spring to see if you do have a problem, you may not have to spray at all. On the other hand, if you do need to spray, control can be more difficult to achieve and more costly. In‐season pesticide application can harm beneficial mites/insects and potentially result in increased spider mites populations.
Certain markets may not buy pesticide treated fruit. Check with your handler if you suspect an issue. The University of California has developed a prune aphid monitoring program for use after bloom to help track aphid populations and know if spraying is necessary. It is available on line at: http://www.ipm.ucdavis.edu/PMG/r606900211.html or in the IPFP Guidelines Binder available from your county farm advisors’ office (University of California Cooperative Extension).
Should a grower use Strategy 1 or Strategy 2? We think the answer depends on the aphid history of each block.
Do you have a consistent problem with aphids? Do you want no concerns with aphids in season? If the answer is "yes" to either of these questions, then a prebloom spray to control aphids should be considered. If you have never seen aphids in the orchard, you might be OK to wait and watch and spray if aphids are found and their numbers begin to build. Pay attention and monitor aphid populations, aphid numbers increase rapidly as weather warms in the spring. Leaf curl aphids will quickly curl leaves making these aphids more difficult to control with contact materials. For pesticide options visit our IPM Web site www.ipm.ucdavis.edu.