An Environmentally-Friendly Alternative for Control of the Citrus Nematode in Arizona

Project Overview

Project Type: Graduate Student
Funds awarded in 2007: $19,746.00
Projected End Date: 12/31/2008
Grant Recipient: University of Arizona
Region: Western
State: Arizona
Graduate Student:
Principal Investigator:
Dr. Patricia Stock
Entomology-University of Arizona

Annual Reports


  • Fruits: citrus, general tree fruits


  • Education and Training: demonstration, display
  • Pest Management: biological control
  • Sustainable Communities: urban/rural integration

    Proposal abstract:

    The citrus nematode, Tylenchulus semipenetrans (Nematoda: Tylenchulidae), is one of the most debilitating citrus pests worldwide. This nematode is an obligate parasite that reproduces only on the living roots of host plants. Nematode females become semi-endoparasites and sedentary by burrowing their anterior ends deep inside the root cortex while the posterior ends remain outside in the soil. They establish feeding sites within the root cortex composed of nurse cells that surround the female nematode head. The posterior portion of the adult female protrudes from the root and is surrounded by a gelatinous matrix in which eggs are deposited. In Arizona, T. semipenetrans was first discovered in 1926 and, since then, citrus growers have faced a continual battle with this citrus pest. Ninety percent of the citrus in the state has been reported to be affected by this nematode. Although this nematode does not kill the trees, it significantly reduces tree vigor, growth and grove productivity. At present very few alternatives are available for control of this nematode. Most chemical nematicides previously considered for control of this nematode (i.e. aldicarb, fenamiphos, DBCP) have been or soon will be removed from the market due to their known toxicity and detrimental health effects to humans, wildlife and soil and groundwater contamination. This situation, and the increasing awareness of environmental and human health concerns associated with chemical nematicides, urges the search for new alternatives of environmentally friendly products for management of this nematode. In this project we propose to assess entomopathogenic nematodes (EPN) as an alternative tool for control of the citrus nematode, T. semipenetrans. Use of EPN as a substitute for chemical pesticides for control of agricultural pests is not a novel idea, but no EPN have yet been tested against this specific plant-parasitic nematode. In order to effectively implement EPN for the control of this plant-parasitic nematode, some basic research needs to be established. Data generated from this study will assist in the subsequent development and application of EPN as an alternative to citrus pest control practices. If proven effective, the consideration of EPN for control of this plant parasite will provide an environmentally safe alternative to traditional chemical control that could be implemented not only in Arizona but in other citrus-producing regions in the country.

    Project objectives from proposal:

    1. To conduct lab experiments to determine the best EPN “species-match” for control of the citrus nematode considering two commercially available EPN S. riobrave (Biovector) and H. bacteriophora (Nemasys) and two Arizona-native EPN (Steinernema sp. ML18 isolate, and Heterorhabditis sp., CH35 isolate). 2. To conduct two field trials in Yuma, AZ considering the two best-performing EPN isolates tested.

    Objective 1. Assays will be conducted on 2-months old rough lemon seedlings reared in containers, in growth chambers at 25 ºC and 30% humidity. Two EPN application times will be considered: a) SIMULTANEOUS: EPN simultaneously applied with citrus nematode, b) AFTER: EPN applied after citrus nematode establishment in the citrus seedlings. For each of these application times the following treatments will be considered: i) EPN aqueous suspensions (2 concentrations: 1,000 and 10,000 IJ/ seedling); ii) EPN-infected cadavers (with an exposure rate of 100 IJ/wax moth). This second approach is based on a study conducted by Shapiro et al (2003). Their study indicated that EPN application with infected cadavers tends to be more efficacious than aqueous applications because the cadaver applications were under less physiological stress (i.e. osmotic stress). Controls will consist of: i) nematode-free citrus seedlings, water added only; ii) T.semipenetrans-infested seedlings, water added only; iii) citrus seedlings with EPN aqueous suspension only; iv)citrus seedlings with EPN-infected cadavers only, and water added. T. semipenetrans-infected seedlings will be inoculated at a concentration of 12,000 J2s/ seedling. A completely randomized design with 12 containers (citrus seedlings) per treatment for each application time will be considered. Eight weeks after EPN inoculation, citrus seedlings will be removed for assessment of T. semipenetrans establishment in the roots and egg production. Data will be subjected to analysis of variance using SAS. Experiments will be repeated three times. The EPN that performs best in controlling the citrus nematode in the growth chamber trials will be considered for field trials.
    Objective 2. Two field trials will be considered. Tests will be conducted in late May-early June and mid August-September 2008. Plots will be arranged in two row beds in a randomized design with 3 experimental treatments and 10 replicates. An in-row variable tree buffer will be established in each trial to prevent treatment interference. Different EPN doses will be considered to assess the effect of this nematode on citrus nematode penetration, egg production, and citrus root growth.
    EPN doses considered will be: 1) low: 54 nematodes/cm2, 2) medium: 108 nematodes/cm2, and 3) high: 532 nematodes/cm2. Nematode suspensions will be premixed in 2.5 gal of water, and each mixture will be added to the tank of a CO2 pressurized hand sprayer, which has flat fan nozzles. Additionally, EPN-infected Galleria mellonella larvae cadavers (infected 2 days prior application day) will be buried in the soil surrounding the drip-line of the tree following procedures described by Perez et al., 2003). Controls will consider water application only.
    Sampling of citrus nematodes will be conducted in both orchards prior to and after (6-8 weeks after application) EPN treatments. Ten soil samples of approx. 100 g will be taken randomly from the orchard. Samples will be processed in a mist extraction chamber to recover and account for Tylenchulus nematodes. Data recovered from both trials will be subjected to analysis of variance using SAS statistical software package.

    Outreach Plan: Results of this study will be presented to several audiences including a graduate student seminar in the Dept. of Plant Sciences Seminar Series during Fall 2007, a poster presentation in the Student Showcase in Fall 2007 to the general audience. Additionally I will give a one-day workshop at the Yuma-Mesa Agricultural Station to growers in conjunction with Extension Researcher C. Sumner.

    Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.