A Retrospective Look at 2023: Navel Orangeworm Damage at Harvest in Almonds

Jhalendra Rijal, Ph.D., IPM Advisor for Northern San Joaquin Valley, UCCE Stanislaus

The rates of insect and plant development are closely related to weather, particularly temperature. Unique weather patterns from wet winter/spring, cooler early summer, and hot late summer of this year influenced almond pests and crop phenology differently compared to previous years. Navel orangeworm (NOW) development was relatively normal until the population exploded during and after hull split. Major hemipteran insect pests, such as leaffooted bug and stink bugs, were “normal” in 2023, with sporadic damage in some orchards (Table 1, Page 1). Ant damage has been minimal.

What might have happened this year with NOW?

· NOW population and hull split timing. The dynamics between the NOW population and crop phenology determine the overall NOW damage in almonds. One of the cornerstones of NOW management is an early Nonpareil harvest to remove the crop before egg-laying by the third flight of NOW occurs. Figure 1 shows the NOW flight activity for the North San Joaquin Valley in 2023. Nonpareil hull split began around the third week of July in the Modesto area – much later than in most years. Because Nonpareil hull split occurred well after the start of the second flight, the first half of the NOW moths in the second flight laid their eggs on last year’s mummy nuts instead of the current season’s nuts. However, the second half of the second NOW flight was able to lay their eggs on current-season Nonpareil nuts. Because it takes longer (1050 degree days) for NOW to complete a generation in mummy nuts compared to only ~700 degree days in current-season nuts, adults emerged from the mummies and current-season nuts at different times. This resulted in heavy egg-laying during an extended period from August 22nd - September 27th. The delayed onset of hull split, coupled with an extended hull split period this year meant that Nonpareil was most vulnerable during this heavy egg-laying period by the third and fourth flights. NOW adults were actively laying eggs throughout September and even the first week of October, which impacted the pollinizer varieties as well.

· Poor orchard sanitation. The exceptionally wet winter/spring of 2023 made mummy sanitation difficult for many growers. These extra mummy nuts provided a large food source for overwintering and first-generation NOW. This ultimately led to increased NOW pressure later in the season.

· Minimalist approach to orchard management. Due to poor almond and walnut prices, some growers may have elected not to mummy shake, apply fewer sprays, or use cheaper materials. Shorter residual materials may not have been sufficient for the protracted hull split this season.

· Nontraditional insecticide spray timing. Since hull split was at least two to three weeks later than usual in 2023, growers who applied sprays at “normal” timing were too early, and residues were insufficient when hull split finally started.

Figure 1. Seasonal navel orangeworm phenology in almond orchards in the Modesto area, 2023.

New issue – occurrence of Carpophilus beetle infesting almonds.

In the last two years, some growers and PCAs have reported feeding injury in almonds by a Carpophilus beetle (Family: Nitidulidae, generally known as sap-feeding beetles). We collected adult beetles and larvae from infested almonds and sent them to CDFA for identification. CDFA has identified this beetle as Carpophilus truncatus, a species that has been infesting almond orchards in Australia since 2013 but was not known to be present in California. In brief, these beetles seem to attack mostly Nonpareil nuts during hull split. Adults chew through the shell, leaving a small, 2-3 mm, circular hole, and lay eggs on the kernel. Adults and larvae feed on the nutmeat, completely converting the kernels into fine white powder mixed with their white frass (Figure 2). Based on initial reports, beetle infestations may have been widespread in many San Joaquin Valley counties, including Stanislaus, Merced, and Madera. We collected samples from three infested almond orchards in Merced and Stanislaus Counties and evaluated the damage (Table 2, Page 3). Carpophilus feeding injury is difficult to differentiate from NOW damage and often occurs in the same nut.

Although NOW and Carpophilus often infest the same nuts, a significant percentage (21-57%) of the infested nuts were from Carpophilus feeding only. So, it is clear that Carpophilus can damage nuts regardless of previous NOW infestation. CDFA is now working to assess the potential impacts of this species and determining what to do next.