Franz Niederholzer, UCCE Farm Advisor, Colusa and Sutter/Yuba Counties;
Luke Milliron, UCCE Farm Advisor, Butte, Glenn & Tehama Counties

Plum aphids (2 species) are the major pest of prunes. Feeding on leaves and stems of rapidly growing shoots, the mealy plum aphid (MPA) causes curling and stunting of leaves, and also secretes honey dew leading to sooty mold on the fruit surface and an increased risk of end cracking in the summer. Severe, uncontrolled MPA infestations can limit return bloom in prunes, reducing crop load the following year. The leaf curl plum aphid (LCPA) also feeds on growing shoots, severely stunting and contorting leaves and also secreting large amounts of honeydew. Fall or winter are good timings to control aphid. So, with harvest over, it’s a good idea to settle on an aphid control plan to protect next year’s crop.

Both MPA and LCPA move out of most prune orchards to summer hosts once prune shoot growth ceases — usually by June at the latest.  [In highly vigorous, young orchards where shoot growth lasts all season, aphids may remain all season long.] Plum aphids move back into orchards in the fall (late September into October) to feed, mate, and lay eggs that will hatch during bloom the following year.

It is very challenging to effectively monitor plum aphid in California prune orchards before eggs hatch. The fall spur sampling protocol includes aphid eggs, but those eggs are scattered in the orchard and only one egg out of 100 spurs triggers a spray treatment. There is a real risk of missing aphid pressure in a block if you only use the dormant spur test results to determine the need for control. [Still, the dormant spur sampling is vital to scale control, so while looking for scale, keep an eye out for aphid eggs – they are a lot easier to see (when present) compared to scale.]

Since accurate monitoring ahead of aphid infestation is hard to do, growers basically have two choices for plum aphid management:

1. Assume you have an annual problem and treat between fall and full leaf out. If you have a history of aphid infestation when dormant sprays are skipped or applied with poor coverage (every-other row by ground or aerial spraying), it’s a good bet you have a significant risk of annual aphid infestation.
2. Wait and watch, treat if aphids show up once shoot growth gets going in the spring. If you haven’t had much or any aphid pressure in the last few years, the ‘wait and see’ approach is a good option. Before you spray, make sure the pesticide sprayed is cleared with your processor/packer.

Both strategies can be effective with the right pesticide materials, spraying and timing. There are advantages and disadvantages to each plan. It’s a grower’s call. Talk with your PCA about options.

The following table shows a range of timings and materials that control aphids, plus options to control other pests commonly controlled with a traditional dormant spray (pesticide plus oil) – scale and peach twig borer (PTB). If you move away from a traditional dormant spray when controlling aphids, make sure to keep those occasional pests under control.

Always read the label & check with your PCA regarding orchard specific rates, materials & timing.

More information on prune pest control:

Dormant aphid control
San Jose Scale control
Peach twig borer control