Assessing Direct and Indirect Interactions between Insect and Plant Pathogens and Their Impact on Insect Herbivores

Project Overview

GW10-004
Project Type: Graduate Student
Funds awarded in 2010: $24,996.00
Projected End Date: 12/31/2011
Grant Recipient: University of Arizona
Region: Western
State: Arizona
Graduate Student:
Major Professor:
Dr. Patricia Stock
Entomology-University of Arizona

Annual Reports

Commodities

Not commodity specific

Practices

  • Education and Training: demonstration, networking
  • Pest Management: biological control, chemical control
  • Sustainable Communities: public participation

    Abstract:

    In Arizona, as in other parts of the U.S, alternatives for chemical control of insect pests are needed, and entomopathogens such as nematodes (EPN) are promising alternatives for pest management. EPN have been shown to control many insect pests, either applied alone or in combination with other microbial or chemical agents. In this study, we assessed the interactions of two Arizona-native EPN species: Heterorhabditis sonorensis (Caborca strain) and Steinernema riobrave (SR-5) in combination with three chemical (imidacloprid, dinotefuran, indoxacarb) and one biological (Bacillus thuringensis subsp. Kurstaki) insecticide. Fourth instar Helicoverpa zea (Lepidoptera: Noctuidae) was used as the insect host. EPN virulence (measured as insect mortality and establishment of infective juveniles or the number of nematode that penetrated the larva) and reproductive fitness (measured as progeny production or the number of nematodes that emerged from the larva) were evaluated, considering the effect of EPN application time. The nature of the interactions (synergistic, additive or antagonistic) was also determined. Our data showed that most of the combinations of EPN and insecticides were more effective in killing H. zea larvae than either agent used alone. Moreover, synergistic interactions were observed between H. sonorensis with Bacillus thuringensis subsp. kurstaki and S. riobrave with indoxacarb. However, while these combinations increased insect mortality, they also reduced significantly progeny production of the nematodes. This aspect may have a future impact when considering recycling of these nematodes in the soil, affecting the long-term effect of EPN application.

    Introduction

    Arizona and Southern California are characterized by a diversity of irrigated desert vegetable crops such as cotton, alfalfa, citrus, melons, lettuce and small grains, which are produced at various times through the year (Anonymous 1987). This crop diversity together with favorable climatic conditions (high temperatures, dry conditions and other abiotic factors) provides an ideal habitat for a number of insect pests (Anonymous 1987). Among these pests, lepidopterans such as beet armyworms, loopers, heliothines, pink bollworms, cutworms and other caterpillars are considered major pest problems in the southwest of United States (Kerns and Palumbo, 2009). At present, management of these lepidopteran pests typically involves a combination of approaches including cultural practices, insect monitoring and chemical or biological pesticides (Bt). However, chemical pesticides still remain the most widely used method for insect control in vegetables (Kerns and Palumbo, 2009). The prevalent usage of chemical pesticides has generated several problems, such as insecticide resistance, outbreaks of secondary pests, decrease of biodiversity, and many other effects of environmental concern (Lacey et al. 2001). For this reason, the search for environmentally-friendly strategies for pest management is imperative.

    One alternative approach is the use of biological control agents, such as entomopathogenic nematodes (a.k.a. EPN) (Gaugler 1999; Grewal et al., 2005). These nematodes are obligate and lethal pathogens of a wide range of insect pests, including lepidopterans, coleopterans and dipterans. The third-stage juvenile of this nematode, also known as the infective juvenile (IJ), is the only free-living stage, and it is responsible for vectoring pathogenic bacteria (Gamma-Proteobacteria, Enterobacteriacea) from one insect to another (Gaugler and Kaya, 1990). These bacteria kill the insect host in a short period of time (24-48h) by massive septicemia (Walsh and Webster, 2003).

    Several studies have shown the efficacy of EPN to control different insect pests when used alone or combined with other pathogens or chemical pesticides. Thus, many insecticides, nematicides, fungicides and acaricides have been tested to determine their compatibility with EPN (Rovesti et al., 1988; Barbosa et al., 1996; Zimmerman and Cranshaw, 1990). Results from these studies are variable, depending on the type of chemical and nematode species studied (Koppenhöfer and Grewal, 2005). For example, the insecticide carbaryl (1-naphthyl methylcarbamate) showed a positive compatibility with Steinernema carpocapsae and Steinernema feltiae (Das and Divakumar, 1987) but a negative compatibility with Heterorhabditis bacteriophora (Zimmerman and Cranshaw, 1990). Moreover, the insecticide imidacloprid (1-(6-chloro-3-pyridilmethyl)-N-nitroimidazolin-2-ylideneamine) had a synergistic effect when applied in combination with H. bacteriophora (Koppenhöfer and Kaya, 1998; Koppenhöfer et al., 2000, 2002) or S. carpocapsae (Alumai and Grewal, 2004) for the control of Cyclocephala hirta LeConte and C. pasadenae Casey (Coleoptera: Scarabaeidae).

    Until now, most of the studies considering combinations of EPN and insecticides have been focused in their efficacy to control insect pests; however, the effect that abiotic factors such as insecticides may have on EPN performance (virulence and reproduction) and recycling in the soil have not been addressed. In this study, we assessed the interactions of two Arizona-native EPN with a selection of reduced-risk chemical and biological insecticides. We chose the corn earworm, Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae), as the insect host to assess interactions between EPN and the selected insecticides. Results obtained from this investigation will contribute to enhancing management strategies for insect control.

    Project objectives:

    We assessed the interactions of two Arizona-native EPN with a selection of reduced risk chemical and biological insecticides. We chose the corn earworm, Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae), as the insect host to assess interactions between EPN and the selected insecticides.

    Specifically, the following objectives were pursued:

    -To assess the effect of EPN application time in relation to the application of the insecticides on insect mortality

    -To investigate the effect of the selected insecticide on nematode virulence (assessed as insect mortality and IJ establishment in the insect host) and progeny production

    – To evaluate the nature of interactions between chemical pesticides and EPN (synergism, antagonism. additive)

    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.