Challenges: Perspectives of Expert Growers

Cover Cropping and Navel Orangeworm Management in Orchards

(Based on grower interviews conducted in 2020-21)

Navel orangeworm (NOW), Amyelois transitella, is a primary insect pest of several nut and fruit tree crops in California, including almonds, pistachios, walnuts, pomegranates, and figs. Here, we focus on the life cycle of navel orangeworm in California tree nut orchards, specifically almonds and pistachios, and discuss the potential role cover crops may play in their management.

Life cycle

Navel orangeworm larvae overwinter in remnant 'mummy' nuts that either remain in the tree canopy or fall to the soil surface during or after nut harvest (Siegel et al., 2008; Wade 1961). In the spring, usually between March and May, the larvae pupate and the first generation of adult moths, also called the first flight, emerges from these mummy nuts. Since there are no nuts from the new crop available yet to infest, the same mummy nuts will serve as hosts for these first flight moths to lay eggs on, resulting in a new generation (i.e., second flight) of navel orangeworm adults that typically emerge from the mummies in late June. By this time of year, new crop almonds are close to hull split, making them vulnerable to NOW attack. As such, this second flight of NOW will largely make use of the much more abundant (and higher quality) new crop nuts, which leads to exponential growth in their population. Third and fourth flight NOW typically occur in early August and early September, respectively, and are significantly larger in population size than the previous two flights due to the greater availability of viable host material. Larvae and pupae that do not emerge from nuts at the end of the growing season overwinter inside unharvested nuts (Caltagirone et al., 1968). In pistachio orchards, as many as 30,000 mummy nuts may be left behind per acre (Siegel et al., 2008), providing plentiful overwintering habitat for navel orangeworm.

Navel orangeworm larvae feeding on almond nutmeat.

Importance of orchard sanitation for navel orangeworm management

While there are natural enemies of navel orangeworm in California, they are often slow to reach population levels that would control the pest to growers’ satisfaction (i.e. to a standard of less than 2% of nuts infested). Therefore, orchard sanitation methods that remove or destroy mummy nuts are seen as an essential navel orangeworm management practice for growers (UC IPM; Wilson et al. 2020). Orchard sanitation measures mainly consist of ensuring mummy nuts that remain in the tree canopy are shaken down to the ground, swept into the orchard middles, and then mowed or tilled to destroy any navel orangeworm larvae or pupae they may contain. While these sanitation practices tend to be successful in almond orchards (Zalom et al., 1984; UC IPM), their effectiveness in pistachio orchards is variable because pistachio nuts, due to their small size and durability, are not as easily destroyed by mowing or tillage and are harder to sweep into the orchard rows (Siegel et al., 2008).

Compatibility of cover cropping with orchard sanitation

The use of cover crops requires growers to think carefully about the timing of orchard sanitation practices to ensure they are compatible with cover cropping. Different growers handle this situation differently. Seth Cooley, walnut grower with Juneco Farm in Solano County, describes mowing the cover crop in his walnut blocks in early spring to achieve orchard sanitation, which essentially terminates the cover crop early, although he would have preferred a later mowing to allow the cover crop to self-reseed. To overcome this challenge, some growers sanitize their orchards shortly after harvest in the fall, a practice that might be easier to operationalize in almonds, which are typically harvested earlier than walnuts and thus allow a longer window of time for winter cover crop planting before it gets too cold. For example, Kim Gallagher of Erdman Family Farms in Colusa County says that after almond harvest, the first thing they do is remove all the mummy nuts before their fall planting of cover crops. Brian Paddock of Capay Hills Orchard in Yolo County has 12.5 acres of almonds and mows his cover crop three to four times throughout the winter cover crop season to facilitate better navel orangeworm sanitation.

Navel orangeworm larvae can overwinter in mummy nuts that remain in the tree canopy.

Cover cropping can also facilitate mechanized sanitization measures by making it easier for machinery to enter orchards, even in wet conditions. Cover crops help mop up excess soil moisture by enhancing water filtration. Their root systems stabilize soil aggregates and create channels where water can flow down into the soil, reducing both runoff and ponding from heavy precipitation (Blanco-Canqui et al., 2015; Joyce et al., 2002). As Kirk Pumphrey, almond grower in Yolo County noted, "wherever there is a cover crop, we never have standing water." For more information on how cover crops influence water management in California orchards, check out our Expert Grower Perspectives article on cover cropping and drought.

Potential of cover crops to reduce navel orangeworm populations

Recent research suggests that cover crops may also reduce overwintering navel orangeworm populations. A study in an almond orchard on the west side of the San Joaquin Valley demonstrated that remnant almond nuts placed into cover crop plots in the winter produced fewer adult navel orangeworm in the spring, potentially due to increased overwintering mortality (Wilson et al., 2022). Navel orangeworm mortality tended to increase the longer mummy nuts lay in the cover crop. While the mechanisms underlying this effect were not identified, one possibility is that characteristics associated with cover crop microclimates (e.g., humidity, temperature, etc.) may speed up the degradation of nuts, making them unsuitable overwintering sites. The study also found some evidence that denser cover crop mixes may lead to more mortality, which could also be related to changes in microclimate conditions (Wilson et al., 2022). While these data are promising, it is unlikely that cover crops alone could substitute for the physical removal and destruction of mummy nuts (i.e. orchard sanitation).

In contrast, another study found no effect of orchard ground cover on the emergence of navel orangeworm from pistachio mummies, though this lack of effect may have been influenced by an abbreviated observation window and/or relatively dry climatic conditions that prevented mummy nuts from adequately degrading in the ground cover (Siegel et al., 2008). Regardless, the mixed outcomes of ground cover on navel orangeworm populations across orchard systems indicates the need for future studies that disentangle when and how cover crops impact the abundance and behaviors of this pest, and for now should not be seen as a substitute for mechanical crop sanitation.

Cover crops may discourage navel orangeworm moths from laying eggs

The San Joaquin Valley study also found that in one year of the two-year study, cover crops reduced springtime egg deposition on remnant nuts on the orchard floor. While the mechanism underlying this effect was not explored, one theory is that the dense ground cover may have interfered with the moths’ ability to locate and access remnant nuts. While this is an interesting and additional impact of cover crops on NOW populations, it is important to note that spring flight NOW primarily deposit eggs onto mummies located in the tree canopy, rather than on the orchard floor. As such, reducing egg deposition onto mummies located on the orchard floor is not likely to have a significant impact on overall NOW populations in the orchard, but the magnitude of this impact should be investigated further.

Furthermore, this effect on spring egg deposition requires that cover crops remain in place much later into the spring than usual. To benefit from this effect, it would be important for orchard growers to allow at least partial stands of their winter cover crops to persist through the spring flight period; if cover crops are fully terminated in the late winter before adult moths emerge—a common practice to minimize frost risk during almond bloom—there will be no ground cover to impede egg laying (Wilson et al., 2022). For more information on reducing frost risk in orchards with cover crops, see our Expert Grower Perspectives article on frost risk. Additional research measuring the effect of different spring mowing programs could shed light on how much vegetation is needed to discourage egg laying (Wilson et al., 2022).

An adult navel orangeworm moth.

In summary, while cover crops can have some influence on NOW winter mortality and spring egg deposition, they should not be considered as a substitute for mechanized sanitation that reduces the overall number of remnant nuts in the orchard. Instead, winter cover crops can be seen as a potentially complementary practice, if they can be established and managed in a way that does not interfere with crop sanitation. For instance, even with the most aggressive sanitation program, some small fraction of remnants will likely remain in the orchard, and the presence of winter cover crops could then possibly help to reduce the survival of the remaining NOW a little further, although further studies will be necessary to better understand this dynamic.