Navel Orangeworm Biofix in a Wet Spring

Sudan Gyawaly, Northern Sacramento Valley IPM Advisor
Franz Niederholzer, UCCE Farm Advisor, Colusa, and Sutter/Yuba Counties


Final wet and cool days are wrapping up as we quickly approach hot and dry weather. The extremely wet winter this year has likely helped growers to kill more overwintering navel orangeworms (NOW) than in warmer and dryer winter years. However, a wet, cold winter will not kill all NOW, and being complacent might result in a bad NOW year. Monitoring the NOW population to make management decisions this year is as important as in the years with low rainfall and warm winters.

The NOW flights in orchards are just picking up in our region, with a few males and a couple of females captured each week as of April 28 (based on six pheromone and six Peterson traps in the Chico and Orland area). Biofix in 2023 at the Nickels Soil Lab in Arbuckle was April 22.

Commonly used NOW monitoring traps in almonds are pheromone traps, bait-bag (Peterson) traps and egg traps. Pheromone and Peterson traps track the male and female moth, respectively. Both traps track seasonal activity and provide information about the beginning, peak, and end of each flight. Egg traps help to detect the beginning of the female egg laying, set the spring biofix, and calculate the heat units (degree days) to determine the potential insecticide treatment timing. Egg traps are typically placed in the orchard by March 15.

Getting the correct date for biofix in a particular orchard is very important to limiting NOW damage. Biofix and degree days are used to predict the start of egg laying for each generation.  Shaking nuts ahead of the start of egg laying of later generations in August and September can reduce NOW damage (nuts on the orchard floor are harder for the female NOW to find and lay eggs on). Especially in the Sacramento Valley, the trick to getting this “free” NOW damage reduction is find the first eggs of the season in the egg traps.

According to UC IPM guidelines, spring egg biofix is marked by two consecutive detections of eggs on most egg traps when checking at least twice a week. However, in the Sacramento Valley, based on the research experience of Dr. Frank Zalom of the UC Davis Entomology Department, the first NOW egg catch is considered biofix. It is important to have traps up early to catch the first eggs and accurately estimate biofix. Details on NOW degree day calculation are available at UC IPM Guideline.


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