Almond Hull Split
Contributed by Layne Wade, Grow West Technical Service Manager
As July approaches a dominant agricultural phenomenon in Northern California is the occurrence of almond hull split (the opening of the almond hull which allows the nut meat to dry out and complete its development). This is an indication that the crop is fully maturing and is the precursor to harvest. It is also the beginning of a very crucial pest control period in almond production, both in terms of reducing insect damage as well as minimizing the effects of disease-causing fungi.
Navel Orangeworm (Amyelois transitella) otherwise known as NOW is perhaps the most damaging arthropod pest of almonds (and pistachios) in California, in large part because they attack the nuts directly and reduce or destroy the value of the infested crop. A large part of controlling this damage is enacting proper cultural activities, such as winter removal of almond mummies (which reduces potential feeding sites for NOW before the current crop can sustain their development). Another effective cultural practice is careful water management during the period approaching and including hull split to encourage the majority of developing nuts to split as quickly and in sync as possible.
These cultural practices often should be followed up by insecticide applications at the beginning of hull split to maximize production of clean, saleable nuts. While the crop is maturing, female moths lay their eggs on the developing nuts, frequently in the direct vicinity of the hull suture (where hull split takes place). For this reason, there are two important considerations to minimize NOW damage. As usual a main factor is applying control materials at the time that correlates with insect target emergence, in this case the caterpillar egg hatch. The second consideration is getting excellent coverage of applied materials. Since the distance the caterpillar travels to get inside an opening hull is very short, its exposure time to an insecticide is also very short. If there is no contact with a control material before the caterpillar gets in the hull, there will be no effect of the application.
In recent years we have seen good control activity from applications of Altacor, Intrepid Edge, Besiege and some other materials applied at hull split. Rotation of these, and other, products is an effective way to manage the pest while minimizing the development of resistance. Making applications as close to actual hull split as possible is also critical as insecticides have finite residual activity, typically in the 2 to 3-week range. It has been very common for applications to be made starting around July 4th (to coincide with Non-pareil development) when actual hull split doesn’t even begin for 2 weeks afterward. Orchards that have extended hull split periods sometimes don’t complete this vulnerable stage for another 3 weeks, meaning there is a large amount of nut exposure when there is no control material present. This can prompt the necessity of a second application.
Careful water management can also aid in condensing the hull split period which enhances the action of applied insecticides. Another benefit of this is minimizing the time that nuts are exposed to germinating fungal spores that create Hull rot. Hull rot impacts the crop over a more long-term period as the fungal infection results in the production of fumaric acid within the limb, which kills shoots and limbs. The more infection points the more and larger limbs that are killed and taken out of production, even for the life of the orchard. Including a fungicide, such as Ph-D, Quash, Luna materials, Quadris Top or others with the hull split sprays has been effective in reducing infections. Of note is the timing of these treatments for Non-pareil almonds, which can coincide with other disease control (such as Alternaria) on pollinator varieties. As always, see your Grow West PCA for specific recommendations.