2020 Navel Orangeworm Trap Catch Update

Recent reports show that 2020 degree day accumulations are shaping up to be similar to that of 2015.

“Traps have been going up early this year because it has been so warm. Heat unit accumulations since January 1 this year are similar to what we recorded in 2015, which was the earliest navel orangeworm biofix in many decades at the Nickels Soil Lab.” Franz Niederholzer, UC Farm Advisor, Colusa, Sutter and Yuba Counties.

A warm spring has meant that growers, who have their navel orangeworm (NOW) traps out early, may be seeing higher than usual adult activity for this time of year. Although this may concern some, NOW flight at this time of year does not always lead to substantial egg laying.

From our experience, NOW activity that occurs early in the season does not always lead to a meaningful second flight that would represent actual risk of crop damage.

Below is a chart of moth captures from a Semios customer in 2019. The solid lines mark dates where NOW activity was recorded. The dashed lines show 1100 DDF from those dates, which is when you would expect the development to complete for the male offspring from mummy nuts.

Note how the flight that occurred in late April (orange) did not result in a flight in early July, while the flight that occurred in June (green) led to a flight in August.

This is consistent with what we’ve seen in both pheromone traps and egg traps. The chart below shows the male catches per day for another customer. The shaded area shows when they found eggs.

Females will lay eggs between one to seven days after emergence, yet in spite of adult activity from the end of March, no eggs were detected until May.

Low levels of egg laying can be expected in early spring because:

• Navel orangeworm take longer to successfully find mates under spring conditions
• Females lay less eggs the longer it takes them to mate

NOW activity is dependent on temperature and light levels. Male NOW will only respond to female calling when the temperature is 52 °F or above, and light intensity is low. Similarly, females will only call if the temperature is 54 °F or above. During the early spring, cooler temperatures mean less hours in the day in which adults are active. This can lead to delayed mating in NOW.

A study by Burks et al. (2011) found that when nightly low temperatures were between 43 °F and 48 °F, only 13% of the females put out were mated on the first night, while 20% of females were mated between 48 °F and 54 °F. After three days, 25% of the females in the first group were mated while 50% of the females in the second group were mated.

Prolonging the time before mating can reduce the number of offspring produced. Landolt and Curtis (1991) found that unmated females will continue to lay eggs at a rate of 15-20 eggs per day. Therefore if mating is delayed, by the time mating occurs, females may have less eggs for fertilization.

Looking at the nightly minimum temperatures in 2019 (Figure 3), cool temperatures may explain why flights that occurred from March to April last season did not lead to subsequent flights. Nightly minimum temperatures were within or below the 43 °F and 48 °F range.

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