Franz Niederholzer, UCCE Farm Advisor, Colusa and Sutter/Yuba Counties
Navel orangeworm (NOW) is the key pest of almonds, causing more income loss due to reject nuts than any other pest. Pesticide sprays, alone, cannot control this pest. A combination of several specific practices has been proven to reduce NOW damage and limit loss of grower income. These practices are winter sanitation, timely harvest and pesticide sprays (where needed). Mating disruption is an additional option that is seeing increasing adoption. The first of these practices on the orchard calendar is sanitation.
Navel orangeworm find food and shelter in unharvested (“mummy”) nuts through the winter and spring, making those mummies a bridge of trouble from one year to the next in your orchard. Multiple research studies show that NOW damage increases with increasing mummy counts the previous winter. Destroying mummy nuts is the first step towards a cleaner crop and improved grower returns through quality incentives from processors. The goal of orchard sanitation is to turn that “NOW bridge” from a four-lane highway (no sanitation at all) into a narrow footbridge.
Orchard sanitation is practiced by removing mummy nuts from the trees by February 1 and destroying (flail mow or disc) all nuts on the orchard floor no later than March 1st. In light of the expected reduced nut price for the 2020 crop, some growers may consider eliminating this key step in NOW management to “save money”. This is a risky move. The Sacramento Valley 2021 crop has a good chance to be lighter than the record 2020 crop resulting in 1) fewer nuts/tree and so higher % damage even if NOW populations stay the same and 2) larger nuts, on average, with the potential for poor shell seal and greater risk of NOW damage. Don’t play Navel Orangeworm Russian Roulette with your next crop. Sanitize.
The following steps are needed for effective orchard sanitation:
- Before January 15, scout an orchard and count mummy nuts in 20 trees per acre. Include all varieties in this count.
- If average number of mummy nuts/tree exceeds 2, sanitation is recommended. The <2 mummies per tree target was developed in the early 1980’s, when industry average NOW damage was around 3.5%, much higher than today’s range of 1.3%. In the high pressure area of the southern San Joaquin Valley, the recommendation is to limit mummies to 1 per 5 trees (0.2 nuts/tree) and 8 or fewer mummies on the ground per Nonpareil tree, based on research from in 2003-2006 in Kern County. However, in 2019, northern California had higher % rejects level than the “high pressure” south San Joaquin Valley. Simply put, the fewer mummy nuts in an orchard, the lower NOW pressure at the start of the season.
- To sanitize, shake trees to remove nuts by January 31. Shaking can remove flower buds as they begin to swell and gain weight in late January/early February. Unless it is an early bloom season and buds begin to swell ahead of normal timing, shaking can be done into early February without reducing yield. Some buds are removed by shaking in early February, but since only 25% flower set = good commercial crop, some bud loss does not mean less crop at harvest. Some growers hand poll after shaking if too many nuts remain.
- Sweep downed nuts into windrows and mow or disc by March 1. Mow slowly (2 MPH) and check for intact nuts after mowing. If some nuts remain intact, mow again. NOW can survive if the nut is intact.
Final thought. NOW overwinter in almond, walnut and pistachio mummies. NOW are also strong flyers, traveling at least a quarter of a mile to lay eggs. If you sanitize, but your neighbor(s) don’t, your orchard is at risk from NOW damage due to mated female moths, ready to lay eggs, flying over from the neighbors. Talk with your neighbors about sanitizing all orchards in the area and removing seedling “wild” almonds along fence lines as these can harbor NOW, too.