2012 was a bad year for navel orangeworm (NOW). This was due to a lot of factors, which include:
- Mild winter temperatures in 2011/2012, leaving a high overwintering population of NOW;
- Dryer in-season conditions in 2011 that made it harder to shake the nuts from the trees;
- Lack of winter rains reduced the ease of removing mummies;
- Poorly timed hull-split sprays;
- and a slightly above average temperatures through the growing season, increasing degree days, which reduced times between NOW generations, increasing populations.
With that in mind, it may be best to think of NOW management as a numbers game. The higher the population in the beginning of the season, the higher the damage expected at the end of the season.
The most effective way to reduce over-wintering NOW populations is sanitation. By removing the tree mummy nuts from the tree and destroying them before mid-March (beginning of moth emergence), NOW damage at harvest can be reduced. These nuts serve as the primary food source for the developing larvae and are the in-season food source for adults and 2nd flight larvae.
The recommended industry practice is to reduce the average mummy nut count to less than two per tree. This recommendation was developed by performing research and reviewing the data collected from plots throughout the San Joaquin and Sacramento Valley. It basically showed that for every mummy nut left in the tree, 1% damage from NOW should be expected. In other words, 5 mummies per tree equals 5 % NOW damage. Therefore, in order to meet the industry standard of 2% NOW damage levels, there should be 2 or less mummies per tree.
Recent research, however, conducted by Brad Higbee (Paramount Farms) and Joel Siegel (USDA-ARS) has shown that a greater reduction of mummy nuts is needed in areas with higher NOW pressure. In Kern County, they have found that the standard for sanitation has to be less than 0.7 mummy nuts per tree in order to keep NOW damage below the industry standard of 2% (Data Table). Interestingly enough, they also found that both the presence and quantity of “ground” mummies influences NOW damage. They concluded that having more than 8.9 ground mummies per tree increased NOW damage above the 2% industry standard (Data Table). These results were drawn from research conducted over a 5 year period reviewing data from plots established on 50 orchards.
By reviewing this data, does that mean everyone across the state needs to sanitize to less than 0.7 mummy nuts per tree? The short answer is “it depends” because insect pressure varies from the south to the north end of the state. The data shows that fewer mummies within the orchard (both tree and ground), the less NOW damage to expect at harvest. Therefore, higher standards of tree and ground sanitation are advised for growers with orchards in high pressure, higher heat unit/degree day areas. Growers in cooler, low pressure areas should still reduce average mummy nut counts to 2 or less per tree and destroy any mummies that are on the ground.