Effect of winter flooding on navel orangeworm and spider mites in almond orchard

S. Gyawaly, J.P. Rijal
 In California, navel orangeworm (NOW) is the primary pest of tree nuts – almonds, pistachios, and walnuts. The fall generation of this pest’s larvae overwinters inside the leftover “mummy” nuts in the orchard. The established cultural practice for managing this pest includes removing mummy nuts from trees and destroying them to reduce the seasonal NOW populations and crop damage. Another key pest of almonds is several species of spider mites which can indirectly cause economic losses by reducing the future crop yield. A hot and dry orchard environment is favorable to spider mite infestation. Intentional flooding of the almond orchard during winter is a new practice being explored in California’s central valley to restore the groundwater level depleted during the previous growing seasons. In addition to groundwater recharge, winter flooding may help reduce pest populations by increasing overwintering mortality. To evaluate the effectiveness of winter flooding for NOW and web-spinning spider mite mortality in almonds, we used an experiment designed to test the effects of groundwater recharge on almond root dynamics and yield. In the flooded orchards, irrigation water was applied 15.25 cm weekly for four consecutive weeks in January. The non-flooded blocks did not receive irrigation water. Evaluation of almond mummy nuts collected from the floors of almond blocks showed that the proportion of live larvae in flooded blocks was lower (33%) than in non-flooded blocks (42%). In addition, in-season nut damage by NOW at harvest was lower (0.56%) in the flooded blocks than in the non-flooded blocks (1.0%). However, this study found that the orchard flooding did not affect in-season spider mite populations. The results are discussed in the context of integrated management of these two major almond pests.
Gyawaly, S. and Rijal, J.P. (2024). Effect of winter flooding on navel orangeworm and spider mites in almond orchard. Acta Hortic. 1406, 395-402
DOI: 10.17660/ActaHortic.2024.1406.56
https://doi.org/10.17660/ActaHortic.2024.1406.56
pest management, Prunus dulcis
English

Acta Horticulturae