Where to Watch for Diaprepes Root Weevil

Daniel CooperPests, Tip of the Week

Figure 1. Left: Channels in the cortex of a structural root caused by Diaprepes abbreviatus larval feeding. Note the callus tissue surrounding the channels. No regrowth of the cortex will occur. Damage to roots is cumulative, increasing until the tree exhibits a restricted ability to acquire water and nutrients. The wounds also increase susceptibility to infection by Phytophthora species. Right: Leaves fed upon by adult weevils typically will appear ragged.

By Larry Duncan

Prior to the introduction of citrus greening disease in Florida, certain conventions applied to the occurrence and impact of diaprepes root weevil in groves across the state. The abundance and damage potential of this major pest were greatest in some Flatwoods habitats, such as parts of the inland East Coast where newly planted trees sustained major damage. By contrast, weevil infestations on the Central Ridge were often not detected for many years until mature trees suddenly declined due to continuous damage to roots by relatively small numbers of weevil larvae. These regional differences resulted from better conditions for weevil population growth and less ability of trees to tolerate root damage in wetter Flatwoods soils, combined with better conditions for native, insect-killing nematodes to prey on weevil larvae in the coarser sands of the Central Ridge.

Unfortunately, by destroying much of the fibrous root system, citrus greening has reduced the ability of trees to tolerate additional root loss, even by the relatively fewer weevil larvae typically encountered on the Ridge. Root weevil integrated pest management employs cultural practices, rootstock selection and insecticides/ovicides when weevils are observed feeding on new leaf flush, mating and laying eggs in the tree canopy. Chemical or physical barriers on the soil surface or insect-killing nematodes can be employed to reduce the numbers of larvae damaging roots.

When weevil populations are abundant, sighting adults in the canopy and the typically ragged leaf margins created by their feeding (Figure 1, right) can signal the need for management. However, when weevils are few, these signs may not be apparent. That is why it is always good practice to take every opportunity to examine the root systems of citrus trees for symptoms that can indicate damage from insects, nematodes orphytophthora. When resetting, replanting or diagnosing problems in groves, take the time to use a simple pump-up sprayer to wash some of the major roots of excavated trees to determine whether the typical root channeling (Figure 1, left) caused by larval feeding indicates the need to manage the citrus root weevil.

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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