RESISTANCE OF CITRUS ROOTSTOCKS AND GLYCOSMIS PENTAPHYLLA AGAINST LARVAL CITRUS ROOT WEEVILS, DIAPREPES ABBREVIATUS, IN LIVE ROOT OR DIET INCORPORATION ASSAYS.

Authors: Shapiro, Jeffrey; Bowman, Kim; Smith, Hunter

Submitted to: Florida Entomologist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/14/1997
Publication Date: N/A
Citation: N/A

Interpretive Summary: Larvae of the root weevil, Diaprepes abbreviatus, damage citrus roots by girdling the root bark, eventually killing the roots and the tree. A test was previously developed to enable discovery of resistant citrus rootstocks or of other plants whose roots would be resistant to feeding by larvae. In this test, whole plants are exposed to feeding by larvae, and root damage and larval growth inhibition are measured. A new diet incorporation test was developed to enable the discovery of inhibited growth of larvae fed on diet containing isolated, macerated roots or chemical extracts of those roots. The roots of a citrus relative, Glycosmis pentaphylla, were found to reduce larval growth in both the whole plant and diet-incorporation tests, in contrast to roots from two commercial rootstocks (Swingle and Carrizo) and two new hybrids (HRS-802 and HRS-896) that are currently being tested as rootstocks.

Technical Abstract: The growth of larval Diaprepes abbreviatus was measured after rearing them on roots of rutaceous seedlings for 35 or 42 days. Larvae were fed on seedlings of two common citrus rootstocks, two new hybrids that are under development as rootstocks, and one citrus relative. Growth on Carrizo or Swingle rootstocks for 42 days averaged 10.3- and 10.2-fold respectively, growth on the citrus hybrids HRS-802 and HRS-896 for 35 days averaged 7.6- and 6.1-fold respectively, and growth on Glycosmis pentaphylla for 42 days averaged 2.5-fold. A bioassay to test for potential phytochemical sources of resistance against the larvae was developed by incorporating finely milled roots into larval diet. Milled root samples were incorporated into a standard semi-defined diet at 5% concentrations (w/v), and growth of larval weevils was recorded following a 31-day feeding period. Roots collected from uninfested control seedlings in the previous experiment were used. On diet containing no roots, mean larval weight increased 16.8-fold, while weights increased 13.9-fold on diet containing roots of Carrizo, 12.0-fold on Swingle, 15.1-fold on HRS-802, 12.3-fold on HRS-896, and only 5.5-fold on G. pentaphylla. Both tests indicate that G. pentaphylla may represent a source of root resistance to Diaprepes abbreviatus, and the diet incorporation tests indicate potential phytochemical or microbial sources of resistance.