Knowing When Hull Split Starts is Critical for Pest Control

almond mummies
The number of mummy nuts are an indicator of NOW populations.

Identifying hull split is critical in an almond integrated pest management (IPM) program. If summer control of navel orangeworm (NOW) is necessary, it is timed with the beginning of hull split and the beginning of egg laying. Hull split also exposes nuts to peach twig borer invasion and hull rot fungus. The longer the nuts remain on the tree after hull split, the longer they are exposed to these invaders.

Determining the exact timing of hull split is complicated. Ripening does not occur in all fruit simultaneously. It begins in the upper southwest quadrant, in the upper and outer parts of the tree, later extending into the inner and lower sections. Nuts at eye level will be less mature than those at the top.

There are two steps you can take to help you identify the initiation of hull split.

  • Check harvest timing for your variety. The start of almond harvest varies from year to year and district to district, but it usually begins in early to mid-August.
  • Check for the beginning of hull split in early July. Use a pole pruner to cut small branches from the top southwest portion of five or six trees in the orchard to see if hull split has begun.

As hull split approaches, keep in mind two things: insect management, including navel orangeworm, peach twig borer, and spider mite, and water management to reduce the incidence of hull rot.

During NOW flight, eggs are laid on the suture on the surface of the nut and inside the split hull. Sprays should be timed for the beginning of hull split if laid eggs are found. If eggs are not found, attempt to time spraying for initiation of egg laying. Be aware that once this occurs, egg laying on traps will decrease.

Navel orangeworm is monitored by counting the number of mummy nuts remaining in trees. If there are two or fewer nuts, treatment for NOW will probably not be needed. Another method of in-season monitoring is to place baited traps in the spring. If in-season sprays are needed, this allows optimal timing. Sprays should be applied during egg laying in May or with hull split.

At hull split, a knockdown insecticide targeting NOW will reduce the population of adults, hatching larvae and eggs. Use of softer, target-specific chemistries such as Altacor, Intrepid, and Delegate will target NOW and reduce the need for miticide application. These chemicals are target-specific, have quick breakdown and less impact on non-target organisms.

It is important to remember that timing of material applied is just as important as the coverage. Susceptible varieties such as Nonpareil can be vulnerable to pest pressure during hull split. When you are in the range of 1 to 5% hull split, you are in the right timing window.

Information provided by Walt Bentley and researchers at The University of California, Davis.