Assessing Direct and Indirect Interactions between Insect and Plant Pathogens and Their Impact on Insect Herbivores
This project’s main goals are to evaluate the potential of two Arizona-native entomopathogen nematodes (EPN) as alternative biocontrol agents in an integrated pest management program in a desert and semi-desert system and to educate growers and the Arizona agricultural community about EPN.
To achieve the main goal of this study, two lepidopteran lettuce-pests (the corn earworm Helicoverpa zea and the fall armyworm Spodoptera frugiperda) have been considered to evaluate the combined effect of EPN and chemical pesticides to control these two important insect pests. Two Arizona-native EPN Steinernema sp. SR-5strain and Heterorhabditis sonorensis Caborca strain are currently being assessed. The neonicotinoid, Imidacloprid (1-[(6-Chloro-3-pyridinyl) methyl]- N-nitro-2-imidazolidinimine), commercially known as Merit® 75 WP, is also being evaluated at different concentration doses.
A. Research Objectives
1) Evaluate EPN efficacy in combination with chemical insecticides in laboratory and greenhouse studies.
2) Conduct field trials to evaluate field efficacy of EPN in combination with chemical insecticides.
B. Education/Outreach Objectives
3) Develop a statewide extension program aimed at raising grower and pest control advisor (PCA) awareness and knowledge of EPN.
4) Conduct stakeholder input sessions to develop information on how to effectively integrate EPN into future IPM programs.
During this first year of the project we focused on objectives A.1. (To evaluate EPN efficacy in combination with chemical insecticides in laboratory and greenhouse studies and B.3 (To develop a statewide extension program aimed at raising grower and pest control advisor (PCA) awareness and knowledge of EPN).
Objective A.1. Early third-instar Helicoverpa zea larvae were reared in the laboratory. These insects were considered in all the experiments tested. H. zea colony was reared at 28C, 12:12h, 80% RH. Two Arizona-native entomopathogenic nematode isolates tested were: 1) Heterorhabditis sonorensis (CH-35 strain) and 2) Steinernema riobrave (SR-5 strain). We tested EPN compatibility with selected chemical pesticides (See Table 1) in tank mixes by assessing nematode viability and infectivity after exposure to various concentrations. We kept EPN at a density of 500 nematodes/ml in insecticide solutions of at 0, 50, 100, 200 mg (AI/L).The mixtures were kept at room temperature (22C) in 125 ml flasks with 25 ml per flask on a shaker at 65 rpm. There were two trials with two replicates per treatment for each trial. After 24h, 200 µl samples from each treatment were taken and placed in a 10 cm petri dish lid to count number of living and dead nematodes. Nematode infectivity was assessed by adding 25 living nematodes to a 30 ml plastic cup filled with 2 g of moistened sand containing a single insect larva, ten cups/insecticide concentration. Total number of nematodes present in each cadaver were accounted. Positive and negative controls were included. Mortality was recorded after seven days, and each cadaver was placed on White traps to account for nematode progeny production. Experiments were repeated twice.
Stats: Mortality data was corrected using Abbott’s formula (Abbott, 1925). Data was square root or arcsine square root transformed before being subjected to analysis of variance and means separation with Tukey’s test. Chemical insecticides’ concentration effects on nematode survival and progeny production in these experiments was examined with linear or quadratic contrasts.
The chemical pesticides and treatments used for all the assays studied are shown in Tables 1 and 2 respectively. For each treatment three blocks were performed and each block included one larva/cup (for a total of ten larvae/ block). The individual cup was considered as the experimental unit for each assay.
Table 1. Chemical tested:
Commercial name Active ingredient
1. Merit®75 WP Imidacloprid
2. ScorpionTM35SL Dinotefuran (35%)
3. Bt Bacillus thuringiensis subsp. Kurstaki
4. Avaunt Indoxicarb
Table 2. Treatments considered:
1. Control: Distilled water
2. Chemical alone
3. Nematode alone
4. Chemical 1st/Nematodes 24h later
5. Nematodes 1st/ Chemical 24h later
6. Nematode an chemical appliedat the same time
Evaluation of the EPN efficacy in combination with chemical insecticides under laboratory conditions.
We determined the lethal dose 50 (LD50) for all chemicals and nematode strains used in this study. Estimation of LD50 was assessed based on the Probit model (Finney, 1971) which uses regression to determinate the dose that kill 50% of the population in study. The LD50 obtained for each chemical studied are shown in Table 3.
Table 3. LD50 for the insects host calculated for each chemical.
Merit®75 WP 0.01 gr/lt
ScorpionTM35SL 0.125 ml/lt
Bt 2.14 ml/lt
Avaunt 0.03 g/lt
According to the results of LD50 obtained for the interaction between the different chemicals and the EPN´S in study, Merit®75 WP and ScorpionTM35SL are not significantly affecting the mortality of the nematodes when both are applied together, even when the highest dose of chemicals were tested. The percentages of mortality obtained with them were of 8 and 10% respectively, which are not significantly different to the percentage obtained in the control.
For Bt, this test was not possible to perform due to the physical characteristic of this product which does not allow a good observation of the nematode under the scope when both are tested together. After finishing the determination of LD50, the next step will be to test the combination of chemical and nematodes at different times of applications (See Table 2) which are scheduled to finalize by the end of February 2011.
A workshop was organized in October 2010, with the objective to help teachers learn more about how to use nematodes in the classroom. About 25 teachers assisted with this activity, where theoretical and practical sections were developed.
Two posters involving different kind of multitrophic interactions with the EPN H. sonorensis were presented from September 2009 until December 2010:
1) Navarro, P. and Stock, P. 2010. Assessing interaction between entomopathogenic nematodes and plant pathogenic fungi: a multitrophic-multispecies study. Organization of Nemat ologist of Tropical America (ONTA) XLI Annual Meeting. Quito, Ecuador.
2) Navarro, P. and Stock, P. 2009. Assessing interactions between the entomopathogenic nematode Heterorhabditis sonorensis sp (Caborca strain) and the plant pathogenic fungi Fusarium oxysporum f.sp. asparagi. II International Congress of Tropical Nematology & 28th Society of Brazilian Nematologist Meeting. Maceio. Alagoas State, Brazil.
Impacts and Contributions/Outcomes
Not applicable at this time. Our data is too preliminary to indicate or assess any impact or outcomes.
Graduate Research Assistant
Entomology, University of Arizona.
1140 E. South Campus DR.
Tucson, AZ 85721
Office Phone: 5206211317