Submitted to: Applied Soil Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/15/2004
Publication Date: 2/1/2005
Citation: Borowicz, V., Alessandro, R.T., Albrecht, U., Mayer, R.T. 2005. Effects of nutrient supply and below-ground herbivory by Diaprepes abbreviatus L. (Coleoptera: Curculionidae) on citrus growth and mineral content. Applied Soil Ecology. 28:113-124. Interpretive Summary: The Diaprepes root weevil (Diaprepes abbreviatus L.) is a pest of many crops in the Caribbean basin. The insect feeds on many different types of plants and is a devastating pest of citrus. The Diaprepes root weevil is the main reason that citrus cannot be grown to any large extent in Puerto Rico. It was discovered in Florida in the early 1960's in an orange grove just north of Orlando and has since spread to almost every citrus producing area in Florida. Recently the pest was discovered in the Rio Grande citrus growing region of Texas. The insect causes the most damage to plants during the larval stages when it feeds on roots. Two or three Diaprepes root weevil larvae can kill a citrus tree by girdling the crown of the root by feeding. Here we have taken two commercially used rootstocks and evaluated the effects of soil nutrition on the Diaprepes root weevils and the trees. We found that the larval feeding reduced both shoot and root masses and the reduction in growth was unaffected by fertilizer concentration. Generally, better-nourished trees produced better nourished (larger) larvae. In regard to effects on possible plant defensive components better-nourished trees had higher levels of defensive enzymes (in roots and shoots), but fertilizer concentrations did not consistently affect total phenolics in roots of either rootstock. Larval feeding variably affected defensive enzymes. Levels of chitinase and glucanase were depressed in roots of Swingle citrumelo and sour orange while peroxidase levels were elevated. These effects were reversed in foliage. The conclusion is that citrus growers cannot compensate for root mass loss due to Diaprepes root weevil larval feeding by raising fertilizer concentrations.
Technical Abstract: Models of plant-herbivore interactions predict a variety of outcomes for the effects of soil fertility on herbivore performance. Nutrient supply can affect the quality of plants as food for herbivores by altering nutrient content or defense chemistry and can also affect the ability of plants to withstand herbivory. Soil fertility effects on tolerance and resistance of citrus to feeding by Diaprepes root weevil (Diaprepes abbreviatus L.) larvae were determined using sour orange (Citrus aurantium) and Swingle citrumelo (Citrus paradisi Macrophylla X Poncirus trifoliata) rootstocks in separate experiments. Sour orange, a cultivar of low resistance to root weevils, and the more resistant Swingle citrumelo, exhibited increased shoot, and the latter showed increased root growth, with increased nutrient supply. Shoot and root masses of both rootstocks were reduced by root weevil feeding and the reduction in growth was unaffected by fertilizer concentration. Mortality of larvae feeding on either rootstock was unaffected by fertilizer concentration, but larval mass was greater on better-nourished sour orange plants. The positive correlation between larval mass and fertilizer on sour orange was consistent with the Plant Vigor Model for effects of plant growth on insect performance. Roots and leaves of better-nourished sour orange and Swingle citrumelo had larger amounts of total proteins than nutrient-stressed plants, suggesting that increased resource supply increased the nutritional quality of roots for weevil larvae. Roots and mature leaves of better-nourished sour orange plants also had increased chitinase, B-1,3-glucanase, and peroxidase activities. The effect of fertilizer was less consistent for Swingle citrumelo. Fertilizer did not consistently affect total phenolics in roots of either rootstock. Fertilizer and herbivory rarely interacted, indicating that induction of defense proteins was not a function of nutrient supply. However, larval herbivory increased root protein content, and in some tissues decreased enzyme activities associated with resistance to microbial pathogens. Insect performance on sour orange, but not Swingle citrumelo, was enhanced by improved plant nutrition despite elevated levels of putative defense proteins. Failure to observe a significant pattern of larval performance with fertilizer on Swingle citrumelo may either be the result of resistance factors or a mite infestation.