Franz Niederholzer, UCCE Farm Advisor, Colusa and Sutter/Yuba Counties
Sudan Gyawaly, UC IPM Advisor, Sacramento Valley

Is winter sanitation really necessary? This question was asked at a recent grower meeting. Our answer is YES, now, more than ever in the last 20 years. This is why…

• Multiple research studies confirm more mummies per tree in the spring results in more navel orangeworm (NOW) damage at harvest.
• Low reject numbers are no longer the norm in the Sacramento Valley. Nut damage levels in the Sacramento Valley are up and now are similar to levels in the San Joaquin Valley where damage was historically higher.
• Each mummy can host up to a dozen NOWs, producing high damage risk from just a few mummies.
• NOW are the major cause of rejects in almonds, but Carpophilus beetle is a new nut pest and overwinters in mummies. Orchard sanitation is the only proven control tool for Carpophilus beetle.

Details:

Research results consistently show that winter sanitation reduces NOW nut damage at harvest. In the early 1980’s, based on research in 15 ranches in the southern San Joaquin Valley, a recommendation of two mummy nuts per tree in February was set. The most recent, comprehensive study on almond orchard sanitation and harvest damage was done in Kern County in 2003-2006. This was a huge study done across 50 ranches and 20,000 acres of conventionally farmed almonds receiving hull split sprays. The researchers concluded that a target of one mummy per 5 trees was needed to deliver field damage less than 2%. (See data in Table 1.)

Reject levels in the last decade are up in all almond growing regions of California. Sacramento Valley growers are as vulnerable as Kern County growers in any given year (Table 2). Careful NOW control, including winter sanitation, where mummy counts show more than 2/tree, is a recommended practice throughout California almond growing districts.

There is no 1:1 ratio of %mummies to %damage (see Table 1). Almond mummies can host multiple worms with reports of up to 12 insects in a single nut. (Every mummy is a potential NOW condo.)

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Timeline for almond orchard sanitation
(https://ipm.ucanr.edu/agriculture/almond/)

January 15: Count mummies in at least 20 trees of all varieties in an orchard. If there are more than 2 mummies/tree (averaged across all varieties) per orchard, sanitation is needed to reduce damage at harvest. If counts show a need, all varieties should be sanitized.

February 1: Remove mummies from all trees by shaking with a harvester and/or hand polling. Count mummies after sanitation to confirm effective sanitation (<2 mummies/tree). More than 2 mummies per tree means risk of unacceptable worm damage at harvest. One mummy in 5 trees is the highest standard for damage reduction, but don’t let perfect be the enemy of good. The basic target is less than two mummies per tree*. Check tree crotches to make sure all mummies get to the ground. The recommended mummy removal timing is before Feb 1. Trees can be sanitized into early February without harming crop yield at harvest, but some bud loss occurred. Growers considering late sanitation (after Feb 1) should test shake and decide if bud loss is acceptable or not, relative to risk of damage at harvest.

March 1: Sweep/blow mummies into the row middles and destroy them by flail mowing. Check the orchard floor after mowing to ensure all mummies are destroyed. If not, mow again until all are crushed.

Two* is the target

*or fewer mummies/tree (average)