Winter 1995 (v7n1)
J.W. Dlott, K. M. Daane, M.P. ]ones and /.M. Peterson.
Adapted ftom Plant Protection Quarterly 4(3):1-5 1994
The inclusion of farmers in the agricultural research and extension process has emerged as a central theme in the development of more sustainable production systems. Though this call for farmer participation has become more widespread, few studies have documented the actual process whereby farmers participate with scientists in defining, implementing, and/or evaluating research. In 1991, we began just such a collaborative grower-scientist pest management research project. The objectives were to develop a research agenda in cooperation with peach growers, implement scientific studies that addressed questions raised by farmers, evaluate the effectiveness of this participatory approach in generating relevant information, and document the overall process. This paper reports on two of these objectives. First, we provide a brief description of the development of the research agenda. Second, we present the results from the on-farm studies.
Developing a Research Agenda
The research program was developed in collaboration with members of the non-profit organization California Clean Growers Association(CCGA) utilizing a methodology called participatory rural appraisal (PRA). PRA combines secondary data review, semi-structured interviews, observation of farm activities, and formal and informal group meetings to identify and evaluate specific needs (Dlott et al. 1994). From this work, we developed a research program for the 1_0-91 season. Our focus was on the peach twig borer (PTB), Anarsia lineatella, which was identified by peach farmers as the key insect pest in their production systems. Farmers further expressed interest in understanding and improving biological and cultural controls of PTB that would work in conjunction with Bacillus thuringiensis (Bt) and pheromone confusion.
Following the 1991 field season, a focus group meeting was held with the I 1 -member CCGA Board of Directors, and four observers: three agricultural scientists with expertise in entomology, agronomy, and sustainable agriculture, and a meeting recorder who took notes as well as made an audio recording. The purpose of the meeting was to evaluate the project and then refine future research activities. The outcome of the meeting was a set of farmer-generated research questions, which included: 1) What beneficial insects are eating PTB? 2) How does weather affect PTB survival? 3) What is the feeding pattern of PTB?
In response to these questions, a series of predator exclusion experiments was conducted in 1992 and 1993. The experiments were designed to identify PTB mortality caused by generalist predators (e.g., ladybird beetles, green lacewings, ants, minute pirate bugs) and climatic conditions (referred to as "abiotic mortality"). The exclusion experiments focused on the impact of generalist predators because recent work by Daane et al. (1993) has shown that parasitoid activity on PTB is relatively low in Fresno and Tulare counties.
Materials and Methods
An experimental plot of 60 three-year-old 'Diamond Princess' peach trees was established in 1992 at a farm in Dinuba, California. A second field site consisting of 100 twelve-year old organically managed 'Flame crest' peach trees arranged 5 rows by 20 trees was established in 1993 at a farm in Kingsburg, California. Insect pest management at these sites included a dormant season oil application for the San Jose Scale (SJS), Quadraspidiotus perniciosus; pheromone confusion for the oriental fruit moth (OFM) Grapholita molests; and, at the Kingsburg site only, three applications of Bt at bloom for PTB. The plots received no applications of insecticides, fungicides, or fertilizers when experiments were underway. The orchard floors at both plots were covered by a mixture of planted cover-crops and endemic plant species.
In each plot, shoots with 3-S lateral shoots located in the top meter of trees were selected. Three treatments were established in a randomized complete block design. Then each shoot was infested with a single PTB larva coinciding with the natural emergence of larvae in the area. The three treatments were:
1) Full exclusion (shoot enclosed in an organdy cage): designed to remove the effect of predation.
2) Partial exclusion (shoot isolated by a barrier of Tanglefoot placed around the base of the terminal shoot): designed to remove the effect of predation by the ant Forinica aerata.
3) Open: designed to estimate the combined effects of predation and abiotic mortality.
Natural enemy species composition and relative abundance were measured by direct observation of open and partially excluded shoots. Generalist predators observed on shoots were tested in laboratory no-choice trials to determine if they fed on PTB larvae. Shoot attack rates, measured by the presence of one or more damaged shoots per larva, were determined by monitoring shoots for damage at 2-3 day intervals. On the last sampling date, shoots in all treatments were removed from trees and dissected in the field using a dissecting microscope. This confirmed previously recorded shoot damage, corrected for damage in earlier sampling, and allowed for the collection of surviving larvae. The stage of recovered larvae was noted. All experiments were terminated when larvae in the partial exclusion treatment were fourth and fifth instars.
Analysis of variance (ANOVA) was used to test for effect of treatment and block on: 1) natural enemy relative abundance, 2) larval attack rate, and 3) larval survival. Treatment means for larval attack rate and survival were compared with Tukey's multiple comparison test (P < 0.05).
Results
Generalist Predator Abundance. Generalist predators observed in this study included: a predatory ant, R aerata, convergent lady beetle, green lacewing species, a minute pirate bug species, and two salticid spiders, Sassicus virus and Thiodina sp. All of the predaceous stages of the observed generalist predators fed on PTB larvae in laboratory nochoice trials. The species composition and relative abundance varied between successive PTB generations and farm sites. E aerata was the only predator observed on treatment shoots in all experiments, and only its densities were significantly higher in the open versus partial exclusion treatments. Relative abundance of all other generalist predators was not significantly different between these two treatments. This indicates that the Tanglefoot barrier significantly reduced predation pressure from R aerata.
Survival of PTB Larvae. Larval survival rates were significantly lower in the open treatment as compared to the partial and full exclusion treatments in experiments conducted in the overwintering generation 1992, overwintering generation 1993, and first generation 1993. F. aerata activity was significantly higher on open shoots accounting for the only significant difference in generalist predator abundance. These results provide direct evidence that PTB larval survival significantly increases when R aerata are excluded from foraging on shoots. The data also suggest that predation by E aerata is the reason for the differences in larval survival among treatments.
Larval survival during the second to fifth instars in the second generation (1993) also was significantly lower for both the partial exclusion and open treatments compared to the full exclusion treatment. PTB mortality in the partial exclusion treatment can be explained, in part, by the abundance of jumping spiders at this site. The Tanglefoot barrier did not significantly restrict their movement, as compared to the movement of E aerata.
Discussion
The predator exclusion experiments provide evidence that predation by F aerata significantly decreases PTB larval survival rates. The Tanglefoot barrier provided an effective means for segregating F. aerata from convergent lady beetles, green lacewings, and minute pirate bugs and the results indicate that these other generalist predators did not account for significant mortality of PTB larvae. In addition, larval survival rates were not significantly different between full and partial exclusion treatments. These results indicate that mortality due to generalist predators other than the ant, E aerata, was minimal. The data from these experiments was put into "life table analysis" and gave similar results, showing that predation by R aerata is the most important overall mortality factor for PTB larvae (Dlott, 1993).
Now that F. aerata has been identified as the most important TB predator, the question remains whether or not this predator can be manipulated to the growers' advantage. To answer this question we must consider the biology of F. aerata. Some ant species aggregate to abundant food sources, such as extrafloral nectar, and once on the plants, also seek prey. F. aerata appears to exhibit such foraging behaviors on peaches in the Central Valley. However, E. aerata also exhibits other less desirable behavior, such as tending honey dew-producing scale and aphid insects. Shorey et al. (1993) reported that F. aerata tended several aphid species in plum trees in Tulare County. Aphid outbreaks appear to be more common in plums than in peaches, a phenomenon that may be linked to this ant-aphid relationship. Some varieties of plum are known to produce only small amounts of extrafloral nectar. Under these conditions, R aerata may switch from foraging on nectar to "tending" aphids, leading to increases in aphid populations.
We are currently working with growers to determine which cultural practices can be used to "conserve" ant colonies. Several observations may be of interest. First, in peach orchards managed without in-season broad spectrum insecticides, two ant species are commonly found, F. aerata and the southern fire ant, Solenopsis xyloni, with F. aerata being the dominant species. Second, it appears that cover crops on the orchard floor may affect ant species composition and numbers by providing alternative food sources (e.g., herbivore prey, seeds or nectar) as well as changing the microclimate and soil texture. Finally, using selective management tactics, such as substituting Bt sprays at bloom for broad spectrum insecticides, are an important component in fostering biological control of PTB. We believe that a better understanding of how cultural practices increase or decrease population levels of ant species could lead to exciting new options in stone fruit integrated pest management.
References
Daane, K. M., G. Y. Yokota &J. W. Dlott. 1993. Dormant-season sprays affect the mortality of peach twig borer (Lepidoptera: Gelechiidae) and its parasitoids. J. Econ. Entomol. 86: 16 79-1685.
Dlott, J. W. 1993. Participatory Research in Sustainable Agriculture: Peach Twig Borer, Anarsia lineatelia Zeller, Biology and Natural Biological Control by Formica aerata (Francoeur) in California Peach Agroecosystems. Ph.D. dissertation, University of California, Berkeley.
Dlott, J. W., M. A. Altieri, and M. Masumoto. 1994. Exploring the theory and practice of participatory research in US sustainable agriculture: A case study in insect pest management. Agriculture and Human Values 11: 126-139.
Shorey, H.H., L.K. Gaston, R.G. Gerber, C.B. Sisk and D.L.Wood. 1993. Disruption of foraging by Fonnica aerata (Hymenoptera: Formicidae) through the use of semiochemicals and related compounds. Environ. Entomol. 22: 920-924,
For more information write to: J. Dlott, UC Berkeley, Department of Environmental Sciences, Policy & Management, 1050 San Pablo Ave., Albany, CA 94706-3106.
(CI-PEST.1 35)
Contributed by Jeff Diott
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