Prevent Navel Orangeworm In Almonds
November 10, 2010 In Northern California, navel orangeworm (NOW) damage was very low in most almond orchards this year. The most important reason for this was removal of mummy nuts from the trees that helped reduce the number of NOW larvae that overwintered. Heavy rainstorms last December through February helped significantly with natural sanitation. If the 2010-2011 winter is dry, you’ll have to do more work to clean the trees before bloom to keep worm pressure low next year. This is especially true in the Southern San Joaquin Valley. Dry conditions in the San Joaquin Valley reduce the natural mortality and increase the survival of overwintering larvae, which results in greater worm pressure the following year.
Cleaning House
Orchard sanitation is the most cost-effective means of reducing NOW damage. This statement has been made over and over but here it is again. We really see benefits from an area-wide effort to clean mummy nuts from trees.
Spraying is costly and each in-season spray for NOW only controls about 60% of the NOW population in the orchard at the time of the spray. Although cleaning the trees of mummies during the winter isn’t cheap, it’s 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.
The reason for this is clear. NOW overwinters as larvae in mummy nuts that are left in the tree after harvest and it is in these nuts that the population carries over into the next season. They emerge in the spring as moths and lay eggs on mummies that are still in the trees at that time. The second generation then puts direct pressure on the new crop nuts at hull split and the third generation will chew the nuts up during the harvest period.
Different Approaches
Let’s compare different approaches to NOW control and the theoretical effect on the surviving population. Assume there are 50 mummies per tree and 30 of them each have one NOW larvae. Half of those are female and each female lays approximately 85 eggs. A normal harvest, about September 1, is halfway into the third generation and for argument’s 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:
First generation:
15 females
x 85 eggs
÷ female
= 1,275 larvae
Second generation:
1275
÷ 2 (half female)
x 85 eggs
= 54,188 larvae
Third 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% control. This reduces the population, but, it is not nearly as good as sanitation, as you will see.
Second generation:
54,188 larvae
x 40% survival after
the spray
= 21,675 larvae
Third generation:
21,675
÷ 2
x 85
= 921,196 larvae per tree to
attack the crop at harvest.
The Results Of Sanitation
Now, look at what sanitation does in comparison: start with the same 30 infested mummies per tree, then, winter clean down to two mummies per tree. One is female, one male.
First generation:
One female
x 85 eggs
÷ female
= 85 larvae
Second generation:
85 larvae
÷ 2
x 85 eggs
= 3,613 larvae
Third generation:
3,613 larvae
÷ 2
x 85
= 153,531 larvae 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 second and third generation worm population by 60%, but sanitation by itself, without spraying, reduced the worm population by 93%! When 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 treatment next season only slows the rate of worm damage increase.
Try to achieve sanitation down to one mummy per tree. Obviously, the larger the cleaned area, the more effective winter sanitation will be. You’ll be helped a great deal by cleaning your almond orchard. If you can get all your neighbors to clean up, too, then the benefits to everyone will be multiplied many times over. Don’t ignore NOW populations that might be overwintering in nearby pistachios, walnuts, or pomegranates. Destroying the overwintering sites in those crops is helpful to everyone, too.
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 neighbor’s after hull split before they can even begin to build up in your orchard and hurt your crop.
Removing Mummies
You’ve got until about February 1 to make sure that mummy nuts have been knocked to the ground. Mummies are most easily removed after fog or rain has kept the trees dripping wet for several hours. Gums soften and the added weight makes the almonds knock easier. In higher rainfall areas of the Sacramento Valley, sufficient rotting of grounded mummies in weed cover often kills most worms. Shredding ensures excellent mortality.
In dry areas of the San Joaquin Valley with little weed cover, removing mummies from the trees, blowing nuts off the berms, and shredding them in the middles is essential for an effective winter sanitation program. Recent research has shown good overwintering NOW survival in dry mummy nuts on the ground in Kern County and even some NOW egg laying on dry grounded mummies has been documented. Once on the ground, nuts on dry, bare soil will require shredding to destroy the NOW population.
Just 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 15 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 the mower.
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