NATURAL REGULATIONS OF ENDEMIC ENTOMOPATHOGENIC NEMATODES

• Sponsoring Institution: National Institute of Food and Agriculture
• Funding Source: SPECIAL GRANT
• Reporting Frequency: Annual
• Accession No.: 0198674
• Grant No.: 2003-34135-14078
• Proposal No.: 2003-05437
• Project Start Date: Sep 15, 2003
• Project End Date: Sep 14, 2005
• Program Code: [AH]- (N/A)

Recipient Organization
UNIVERSITY OF FLORIDA
G022 MCCARTY HALL
GAINESVILLE,FL 32611

Performing Department
CITRUS RESEARCH & EDUCATION CENTER, LAKE ALFRED

Non Technical Summary
The key biotic and abiotic factors that regulate populations of entomopathogenic nematodes (epn) used for biological control of insects are largely unknown. A better understanding of how epn populations are regulated will help us understand their limitations and exploit opportunities to increase their effectiveness. This project will investigate putative biotic and abiotic factors that regulate population densities and species diversity of entomopathogenic nematodes in soil.

Animal Health Component

25%

Research Effort Categories

Basic

75%

Applied

25%

Developmental

(N/A)

Classification

Knowledge Area (KA)	Subject of Investigation (SOI)	Field of Science (FOS)	Percent
215	3130	1070	100%

Knowledge Area
215 - Biological Control of Pests Affecting Plants;

Subject Of Investigation
3130 - Nematodes;

Field Of Science
1070 - Ecology;

Keywords

antagonism

biological control (insects)

competition

soil texture

nematodes

entomophilic nematodes

fungus diseases (animals)

edaphic factors

natural enemies

bacterial diseases (animals)

seasonal variation

nematode ecology

insect parasites

host parasite relations

mortality

diaprepes abbreviatus

prevalence

population density

nematode population

insect population

species diversity

Goals / Objectives
We will determine whether endemic entomopathogenic nematodes are regulated by nematophagous fungi, phoretic ectoparasitic bacteria, and/or competition with free-living bactivorous nematodes. We will investigate physical and chemical edaphic conditions that correspond to the prevalence and diversity of endemic EPN.

Project Methods
We propose to investigate the role of natural enemies and competitors of entomopathogenic nematodes (EPN) in regulating the seasonal prevalence of EPN. We have shown that endemic EPN are key natural mortality factors of subterranean stages of the weevil pest of citrus, Diaprepes abbreviatus. The incidence of EPN-induced weevil mortality was seasonal during two years. An increase in EPN prevalence was consistent with a density dependent response to increasing weevil abundance in early summer each year. However, EPN prevalence declined from mid-summer through autumn each year, despite favorable conditions of temperature and moisture and evidence of continuous insect recruitment into the soil. Therefore, we propose to test the hypothesis that endemic EPN in this system are regulated by density dependent agents such as nematophagous fungi and/or free living bactivorous nematodes that compete with EPN for resources in the insect cadaver. We anticipate that the results of this study will enable more efficient intervention with insect management tactics to complement significant, but periodic, natural control of this insect pest. We also propose to leverage an ongoing survey of the diversity and prevalence of EPN in Puerto Rico by investigating the influence of soil texture and chemistry. Several lines of research in Florida provide evidence that the diversity and prevalence of endemic EPN and the efficacy of commercially produced EPN are highly dependent on the percentage of medium-coarse sand in soil. We will obtain texture, pH, and elemental analyses of samples obtained during a T-Star-funded survey of EPN in Puerto Rico (Drs. Byron Adams and Roberto Vargas, co-PD). We will use multivariate statistical methods such as principle component analysis and stepwise multiple regression to reveal relationships between abiotic characteristics of soil and diversity and prevalence of EPN. We anticipate that our results will enable better prediction of soil characteristics necessary for the profitable use of EPN. Additionally, the results will enable us to better understand the role of edaphic factors on the regulation by endemic EPN of insect population density and spatial pattern. The studies in Florida and Puerto Rico are complementary in that they will significantly increase our understanding of biotic and edaphic factors that regulate the behavior of EPN populations. The research conforms with the first and second objectives of the current T-Star action plan.

Progress 09/15/03 to 09/14/05

Outputs
An exotic entomopathogenic nematode (EPN), Steinernema riobrave, was applied in 2004 beneath trees in a Florida citrus orchard to control soilborne larvae of the weevil Diaprepes abbreviatus. Within two weeks, in treated compared to untreated plots, there were significant increases in mortality of sentinel weevil larvae, prevalence of the nematophagous fungi Arthrobotrys sp. and Dactylaria sp., and prevalence of free-living bactivorous nematodes in cadavers of sentinel weevils. Six weeks following treatment, prevalence of EPN declined in treated compared to untreated plots and significantly fewer sentinel weevil larvae died in treated plots. This non-target effect supports our previous report (Duncan et al., 2003, J. Nematology 35:178-186) that EPN augmentation can subsequently and temporarily reduce the level of natural biological control by endemic EPN. Two EPN augmentation events at this site in 2005 also increased the prevalence of nematophagous fungi temporarily, but did not result in subsequent suppression of biological control by endemic EPN. The spatial patterns of the prevalence of endemic steinernematid EPN and nematophagous fungi were inversely related to one another. Temporal patterns of the prevalence of these two groups of organisms were consistent with density dependent regulation of steinernematid EPN by the fungi. Prevalence of heterorhabditid EPN was unrelated to that of nematophagous fungi. In both 2004 and 2005, application of composted manure as a mulch beneath trees decreased the prevalence of nematophagous fungi, increased the prevalence of endemic EPN, and increased the mortality of sentinel weevil larvae. In the laboratory, addition of S. riobrave (exotic) or S. diaprepesi (endemic) to soil from the orchard increased nematophagous fungi and increased the mortality of both S. riobrave and S. diaprepesi that were added to the soil five days later. The effect did not occur in soil that was air-dried to disrupt fungal activity prior to the experiment. These effects were less pronounced when the soil was pretreated with the endemic EPN Heterorhabditis zealandica.

Impacts
These results support the hypothesis that EPN augmentation may temporarily adversely affect the endemic EPN community by increasing the numbers of density-dependent antagonists of nematodes. The experiments also demonstrate that augmenting the endemic EPN community with appropriate EPN species and use of some types of mulches may mitigate non-target effects. They demonstrate the importance of timing EPN augmentation events based on patterns insect recruitment into the soil. If a major period of neonate hatch and entry into soil occurs soon after an EPN augmentation, growers should be aware of possible non-target effects that may hinder biological control of the new insect cohort. A second augmentation event may be required to achieve satisfactory management in such cases. Fortunately, recruitment patterns of D. abbreviatus neonates are somewhat predictable, so that EPN augmentation can be scheduled to avoid preceding weevil recruitment in the majority of cases.

Publications

• Barbercheck M. and L. W. Duncan. 2004. Abiotic factors. In: Nematode Biology. R. Gaugler and B. Bilgrami, Eds. CAB International, St. Albans, UK, p. 309-343.
• El-Borai, F. E., L. W. Duncan, and J. F. Preston. 2005. Bionomics of a phoretic association between Paenibacillus sp. and the entomopathogenic nematode Steinernema diaprepesi. Journal of Nematology 37:18-25.
• Georgis, R., G. Belair, L. W. Duncan, P. S. Grewal, A. M. Koppenhofer, L. A. Lacey, M. Samish, P. Torr, and R. W. H. M. van Tol. 2005. Successes and failures of entomopathogenic nematodes. Biological Control (in press).
• Shapiro-Ilan, D. I., L. W. Duncan, L. A. Lacey, and R. Han. 2005. Orchard crops. In: Nematodes as Biological Control Agents. P. Grewal, R-U Ehlers, and D. Shapiro-Ilan, Eds. CAB International, St. Albans, UK (in press).

Progress 10/01/03 to 09/30/04

Outputs
A two-year experiment was initiated in a citrus orchard in central Florida. Two factorial treatments consisted of application of entomopathogenic nematodes (Sr = Steinernema riobrave) and application of a mulch (composted horse manure). We tested the hypothesis that endemic EPN are regulated in a density dependent manner by competitor nematodes (Duncan et al., 2003b) and fungal predators. We further hypothesized that application of exotic epn would increase the prevalence of nematophagous fungi, and application of manure mulch would reduce the prevalence of these fungi. In year one of the experiment we demonstrated: 1. Application of exotic EPN increased mortality of sentinel insect larvae in soil and prevalence of EPN two weeks post-treatment, but deceased mortality of sentinel insect larvae and prevalence of EPN six weeks post-treatment. 2. Application of exotic EPN increased the prevalence of the nematophagous fungi Arthrobotrys sp. and Dactylaria sp., and increased the prevalence of free-living bactivorous nematodes in cadavers of sentinel insects. 3. Application of manure mulch decreased the prevalence of nematophagous fungi, increased the prevalence of endemic EPN, and increased mortality of sentinel insect larvae. We further demonstrated in the laboratory: 1. Mortality of both Sr and the endemic EPN S. diaprepepesi (Sd) when added to soil is significantly greater if Sr is added to the soil 5 days previously. The effect does not occur if soil is air-dried prior to the experiment. 2. Heterorhabditis zealandica (Hz) mortality is unaffected by pretreatment with Sr in either raw or air-dried soil. 3. The prevalence of the nematophagous fungi Catenaria sp., Arthrobotrys sp. and Dactylaria sp. is increased by application of Sd and Sr to soil, but not by application of Hz. The data support the hypothesis that EPN augmentation may adversely affect the endemic EPN community by increasing the numbers of density-dependent antagonists of nematodes. They also support the possibility that heterorhabditid species may be less affected by the antagonists than steinernematid speces due to protection afforded by the cuticular sheath retained by heterorhabditids.

Impacts
By definition, augmentation biological control attempts to increase the population density of an organism beyond the equilibrium density. The subsequent responses of density dependent antagonists and competitors create a high probability of non target effects. Consequently, a better understanding of the post-application biology of entomopathogenic nematodes is needed to optimize the efficacy of this important biological control tactic. Use of manure mulches in combination with EPN augmentation and selection of appropriate EPN species to minimize the impact of hyperparasites are two methods that merit futher investigation to improve the outcome of biocontrol with EPN.

Publications

• No publications reported this period