Efficacy of a Hull Split Spray vs. Sanitation for NOW Control

Joseph Connell, Farm Advisor Emeritus, Butte County

Navel orangeworm (NOW) is most effectively controlled with the cultural practice of winter sanitation.  Winter shaking almond trees to remove mummy nuts has proven to decrease next year’s NOW damage better than any other approach. The reason for this is clear.  NOW overwinters as larvae in mummy nuts left in the tree after harvest and it is in these nuts the population carries over into the next season.  Adult moths emerge in spring, mate, and lay eggs on mummies that are still in the trees as the females can’t find the new crop nuts until hull split. The second generation will then put direct pressure on the new crop nuts at hull split and the third generation will chew the nuts up during the harvest period.

The way almond prices have been going recently, there’s no doubt that everyone is going to have to spend dollars as wisely as possible for the foreseeable future. Although cleaning the trees of mummies during the winter isn’t cheap, it is the method of NOW control where you clearly get the most bang for the buck spent. We’re really playing a numbers game here, and this is one practice that is stacked in our favor by the biology of this pest.

For example, let’s assume a potential of 50 mummies per tree and 30 of them each have 1 NOW larvae.  Half of those are female, and each female lays approximately 85 eggs. At harvest in late July we’re into the third generation, and for arguments sake let’s assume there’s no natural mortality.

Look at what could theoretically happen to the worm population in one Nonpareil tree with 30 infested over-wintering mummies and no control:

  • 1st generation: 15 females x 85 eggs/female = 1,275 larvae
  • 2nd generation: 1,275/2 (half female) x 85 eggs = 54,188 larvae
  • 3rd generation: 54,188/2 x 85 = 2,302,990 larvae per tree at harvest!!!

(Thankfully, there IS natural mortality or else we’d be knee deep in worms!)

Now, look at the impact of a hull split spray aimed at the second NOW generation. We know that sprays give at best about 60 percent control.  This reduces the population but is not nearly as good as sanitation as you will see.

  • 2nd generation: 54,188 larvae x 40% survival after the spray = 21,675 larvae
  • 3rd generation: 21,675/2 x 85 = 921,196 larvae per tree to attack the crop at harvest.

Now, look at what sanitation does in comparison.  Start with the same 30 infested mummies per tree, then winter clean down to 2 mummies per tree.  One is female, one is male.

  • 1st generation: 1 female x 85 eggs/female = 85 larvae
  • 2nd generation: 85/2 (half female) x 85 eggs = 3,613 larvae
  • 3rd generation: 3,613 larvae/2 x 85 = 153,531 larvae per tree at harvest.

(If you can beat the 3rd generation by an early harvest you’re even further ahead.)

So, a hull split spray reduced the worm population by 60 percent, but sanitation by itself, without spraying, reduced the population by 94 percent! When more NOW larvae make it through the winter, more egg laying will occur next season regardless of what else you do. In relation to the number of mummies left in the tree, expensive chemical treatments next season will only slow the rate of worm damage increase.

If you have scarce dollars to spend on NOW control, spend them this winter when they will do the most good in a sanitation program. If the entire neighborhood works at this, the positive effect will be multiplied many times over for everyone. If you’ve got neighbors that don’t seem to get it, cleaning your orchard will still be a tremendous help to you. If you have no mummies, the first generation in the spring won’t be able to build up and establish a population in your orchard. You’ll benefit since they’ll have to fly in from the neighbors after hull split before they can begin to hurt your crop.

Be sure to finish the job by destroying the infested nuts once they’re on the ground. Mow and shred the mummies next spring before March 1st so NOW moths don’t have a chance to emerge. When you’re enjoying mowing during bloom next spring, take personal satisfaction in seeing the chips and pieces of almond fragments and mangled worm parts fly out from under your mower!


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