By Larry Duncan
Soilborne larvae that feed on citrus roots are the damaging life stage of Diaprepes abbreviatus and Pachnaeus spp. (bluegreen) root weevils.
Newly developed adult weevils, which emerge throughout the warm months of the year, also occur in soil. A peak emergence of adults occurs at some point each spring or early summer. Less frequently, a smaller emergence peak also occurs in autumn. Peak emergence times differ between groves and years, regardless of location in Florida.
The large cohort of young adults at peak emergence provides an ideal management target, because within a week after emergence these pests feed on new flush, mate and begin laying eggs in the tree canopy. Treating adults at this time with insecticides combined with egg sterilants (https://edis.ifas.ufl.edu/publication/CG006) is the most efficient way to reduce the numbers of larvae that will eventually fall to the soil to begin feeding on roots. Focusing insecticide use primarily at spring emergence, rather than periodically during the year, also helps reduce the exposure of non-target, beneficial insects.
The are no practical methods to measure root weevil larvae in soil. However, the presence of the larvae is easily detected by the characteristic channeling they cause on structural roots. These roots should always be examined when resetting or replanting trees.
Adults can be monitored visually, by scouting the trees or with the aid of traps. When weevil populations are large, the adults are readily apparent in the tree canopies during peak emergence, especially in young trees. They are far less apparent in mature trees if the infestation level is not especially high. In such cases, adults may not be detected until trees suddenly decline and longstanding, cumulative damage to the roots is discovered. However, the characteristic ragged margins of leaves fed upon when young and tender should alert growers to the presence of root weevils in the grove.
Regular scouting, especially in early morning, should be sufficient to reveal the presence of these pests. Tedders traps or ground traps can also be placed beneath tree canopies and regularly monitored. Weevils mistake the black base of the Tedders trap for a tree trunk, which they climb until entering a boll weevil trap. Tedders traps capture weevils when moving between trees, migrating seasonally into groves from other food sources, as well as during peak emergence. A ground trap only recovers adults that have emerged from the soil beneath the wire cage.
Neither trap is especially efficient; large numbers must be used in groves when conducting research. However, if monitored regularly throughout the spring and early summer, as few as a dozen traps can help detect peak emergence of the pests as part of a biorational management program.
Larry Duncan is a professor at the University of Florida Institute of Food and Agricultural Sciences Citrus Research and Education Center in Lake Alfred.
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