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Project description

Elucidating mechanisms underlying the suppressiveness of composted organic yard waste towards pupating avocado thrips, Scirtothrips perseae, in avocado orchards. (02XA001)
Program Exotic Pests and Diseases Research Program
Principal
investigators
M.S. Hoddle, Entomology, UC Riverside
M. Brownbridge, Entomology, University of Vermont
P. De Ley, Nematology, UC Riverside
Host/habitat Avocados; Tree Crops
Pest Avocado Thrips Scirtothrips perseae
Discipline Entomology
Review
panel
Agricultural Systems
Start year (duration)  2002 (Two Years)
Objectives Determine effects of abiotic factors (humidity and temperature) on avocado thrips survival in autoclaved mulch.

Evaluate effects of biotic factors (natural enemies, i.e., micro-arthropods, fungi, nematodes, originally recovered from composted organic yard waste, mulch) on avocado thrips.

Determine if guilds of natural enemies suppress thrips in autoclaved mulch more effectively than individual species.

Project
Summary
Avocado production in California has been severely affected by Scirtothrips perseae, an invasive thrips native to Mexico and Guatemala. This exotic pest causes eight to eleven million dollars in losses per year to the avocado industry. Composted organic mulches used for biological control of avocado root rot can significantly reduce adult thrips emergence rates from soil in comparison to non-mulched trees. Mulch harbors a diverse micro-arthropod, entomopathogenic fungi and nematode fauna that may be antagonistic towards thrips larvae and pupae.
Final report Evaluation of Microarthropods isolated from mulch: Currently, we have two cultures of predatory mites in the laboratory: (1) Hypoaspis sp. and (2) Amblyseius sp. both genera are known predators of thrips larvae in subterranean habitats. These mites are being evaluated for predation toward avocado thrips.

Evaluation of fungi isolated from mulch: More than 800 fungal isolates were recovered from Californian avocado orchard mulch and soil samples, primarily species of Beauveria bassiana, Metarhizium aniospliae, and Verticillium llecanii, that are known insect pathogens. These have now been placed in long-term, low-temperature storage, preserving them for future use. Bioassays on more than 280 isolates have been completed, using western flower thrips (WFT). Selected strains were tested against field collected avocado thrips; results showed good correlation between activity against WFT and avocado thrips, validating the use of WFT as a surrogate test host. Data from WFT assays is being used to select the best isolate(s) for testing in the mesocosm studies.

Evaluation of nematodes isolated from mulch: We have begun testing the nematode Steinernema feltiae, for its pathogenicity toward avocado thrips. So far, the results of laboratory assays have not given a clear answer on the pathogenicity of S. feltiae toward avocado thrips. Dead thrips are recovered from exposure experiments, but we do not find nematodes inside these dead thrips. Larger experiments are planned (including time lapse videography of nematodes and thrips larvae in experimental wells) to test whether the nematodes might be killing thrips without fully entering or permanently remaining inside their bodies.

Mesocosm evaluations: Work is being repeated due to design flaws that were encountered last year. We will be examining six different treatments for controlling avocado thrips pupae in much: (1) predatory mites added to sterilized mulch, (2) entomopathogenic fungi added to mulch, (3) entomopathogenic nematodes added to mulch, (4) mite, fungi, and nematodes combined, (5) sterilized mulch that lack natural enemies as a control treatment, and (6) fresh avocado leaves and no mulch will act as the overall control for the control to elucidate any adverse effect from the mulch itself.

Field evaluations: After 16 months of bi-weekly field evaluations at ACW Ranch in De Luz, the field experiments evaluating mulch impact on avocado thrips population dynamics in a commercial orchard were terminated before the intended date. Unfortunately, we encountered repeated problems with the orchard manager and the cultural management of our 1.5 acre high density Hass avocado plot, such as: (1) regular random roughing of study trees due to suspected poor performance and the said need to optimize economic profitability, (2) laying of mulch in control plots to improve plant vigor to levels comparable to that seen in the treatment plots, (3) removal of flagging tape that delineated replicated plot treatments, (4) broad spectrum insecticide applications to kill nonthrips pests that negatively affected thrips populations, and (5) severe winter storms that hindered access to this remote orchard. Despite early closure, the field trials generated large amounts of data, and we are currently in the process of analyzing these.

Second-year
progress
Currently we have two cultures of predatory mites in the laboratory: (1) Hypoaspis sp. and (2) Amblyseius sp. both genera are known predators of thrips larvae in subterranean habitats. These mites are being evaluated for predation towards avocado thrips.

Over 800 fungal isolates recovered from mulch and soil samples and the majority of those isolated are recognized insect pathogens such as Beauveria bassiana, Metarhizium anispoliae and Verticillim lecanii. So far, 150 of the 250 insect pathogenic isolates selected for testing have been evaluated against western flower thrips; our surrogate test host. Some fungi show little or no activity against the test insect, while others appear to be highly virulent. These tests have enabled 18 active isolates to be identified for further testing against avocado thrips.

We have begun testing the nematode Steinernema feltiae, for its pathogenicity towards avocado thrips. So far the results of laboratory assays have not given a clear answer on the pathogenicity of S. feltiae towards avocado thrips. Dead thrips are recovered from exposure experiments but we do not find nematodes inside these dead thrips. Larger experiments are planned (including time lapse videography of nematodes and thrips larvae in experimental wells) to test whether the nematodes might be killing thrips without fully entering or permanently remaining inside their body.

Ten replicates of six different substrates have been tested in the mesocosms for pathogenicity towards avocado thrips larvae. We did not demonstrate any significant differences between treatments. Problems have been identified and the experiment is to be repeated in March 2004.

First-year
progress
Composted organic yard waste laid under avocado trees can suppress emergence of adult avocado thrips out of the soil by approximately 50% in comparison to non-mulched trees. The mechanisms underlying observed suppression are undetermined. In the laboratory we are investigating the potential antagonistic role of predatory microarthropods, fungi and nematodes towards pupating avocado thrips. This work is at the very preliminary stages at present. The Hoddle lab is assaying sterilized and field-collected mulch and naturally occurring leaf material. The mesocosms in which these studies are to be conducted have been made, and field sites have been selected for the collecting of thrips larvae to seed the mesocosms. Soil nematodes are currently being identified and compared between treatments with or without mulch. Twenty-one genera have been isolated to date from mulched trees, versus only eleven from non-mulched trees. Molecular and morphological species identifications are in progress. One isolate from mulch was identified as Steinernema feltiae and is most likely to inhibit thrips pupation; cultures of this species are being established and other species that may be entomopathogenic for use in infectivity tests and mesocosm trials. Insect-pathogenic fungi recovered from soil and mulch samples during 2000-2002 have been identified and preserved. The predominant species isolated (>90% of isolates) is Beauveria bassiana, but two distinct morphotypes have been recovered; one seems to be better adapted to colonize mulch/soil rather than just the insect host alone. Representatives of both morphological forms will be included in the insect assays, along with selected strains of Metarhizium anispoliae, to provide information on their relative virulence.

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