Development and Implementation of Trap Cropping Strategies for Control of Hemipteran Pests in Pistachio Orchards

Development and Implementation of Trap Cropping Strategies for Control of Hemipteran Pests in Pistachio Orchards

Summary

Objectives

1. Field evaluate the potential of selected trap crops to reduce hemipteran pest damage in pistachio orchards.
2. Disseminate the information developed on trap crops to pistachio growers.

Abstract

Many California pistachio growers plant cover crops to increase soil fertility, improve water penetration, and reduce pest problems. However, there is evidence that some cover crops also suppress hemipteran pest damage. Feeding by many hemipteran pest species can cause pistachio nut damage known as epicarp lesion and kernel necrosis. The potential use of trap crops to keep pests out of the pistachio canopy was studied in nine orchard blocks near Madera and Merced, California. At each site, six trap crop species (alfalfa, barley, bell beans, rose clover, Langudoc vetch, and mustard) were planted. Orchard-floor vegetation was sampled at biweekly intervals, with measurements made of plant species, height, and phenological stage. Insects were monitored, with biweekly sweep-samples taken from bud-break through harvest. Cluster damage was monitored in selected blocks, with counts of epicarp lesion and total nuts taken from fruit set through harvest to determine levels of epicarp lesion and kernel necrosis.

While there was an increase some beneficial insects, there was little change in pistachio nut damage between treatments. Results show that many hemipteran pest species overwinter inside as well as outside the orchard, reducing the importance of trap crops for migratory pests. Calocoris, red-shouldered stinkbugs, and flat green stinkbugs were all found overwintering in the orchard and were the first bugs to attack new fruit. Furthermore, the pistachio tree compensated for early-season damage by Calocoris and red-shouldered stinkbugs (i.e., there was no decrease in yield). In contrast, leaf-footed bugs, were not found overwintering in the orchard, and a late-season migration of large numbers of leaf-footed bugs, after the trap crops had been plowed down, required insecticide treatments in all plots, emphasizing the seasonal limitation of trap crops. There were differences in the numbers of hemipteran pests in the different trap crop species. Significantly more Calocoris were collected in mustard rows. However, there were few differences among trap crop species or clean-cultivated controls in levels of late-season epicarp lesion.

Studies of hemipteran pests and trap crops led to two additional areas of research. First, cage studies of hemipteran biology and ecology showed considerable differences in feeding damage and phenology between different kinds of stinkbugs. Results indicate that hemipteran species occurring later in the season warrant greater concern than others because of early-season crop compensation. Moreover, the results imply that trap crops, which are plowed under in May or June, will have little benefit on the more damaging late-season hemipteran pests. Second, we investigated the relationship between trap crops, hemipterans, and the pistachio disease caused by the fungus Botryosphaeria dothidea (Bd). Over 12,000 insects were collected from trap crops and plated on an agar medium for the presence of Bd. While few insects tested positive for Bd (<0.2%), we did find spores on Calocoris, Lygus, and a few other small bug species, but not on any of the stinkbug species that were most commonly collected in the ground covers. A series of studies followed. A summary of key findings are the following: 1) hemipteran pests can carry Bd spores for three to ten days, with wide differences between species, 2) hemipteran pests most likely acquired Bd spores inside, not outside, the orchard, and spores can accumulate on trap crops (Bd was most commonly cultured from insects collected in the trap crops), 3) stinkbug feeding facilitates fungal infection by providing germination sites on the plant surface, and 4) evidence suggests that hemipterans rarely carry Bd spores between orchards.

Dissemination of Findings

We had many factors working towards rapid dissemination. First, the research was conducted in a grower participatory format. Second, we worked with the Areawide IPM Farm Advisor and Farm Advisors in Central Valley pistachio growing regions. Third, presentations were made at symposia sponsored by the California Pistachio Commission. Grower-oriented field demonstrations were also conducted in the second year of the study (1999). Finally, because pistachio growers are greatly concerned about the fungal disease (Bd) and hemipterans were implied as between orchard vectors there was great anticipation of our findings. Attendance at each of four grower symposia ranged from 200 to500 people. Publications in popular and referred journals are planned and will disseminate information outside of California. Currently, the SARE sponsored information is being included in a “Pistachio Production Manual.

Potential Benefits

Pistachios are one of California's fastest-growing agricultural commodities. Currently, California pistachios have a worldwide export market and an annual crop value of over $140,000,000. Because pistachio trees grow well in poor soils and require relatively little water, they can be planted in regions where the sustainable production of alternative crops (e.g. tomatoes, cotton) is not as feasible.

Though once considered virtually pest-free, California pistachios are now attacked by a variety of insect pests. This research provided valuable new information on hemipteran pest biology and ecology. More important, correlation of insect pest density in the trap crops with standard sampling methods (e.g., beating tray) and crop damage clearly showed the inadequacies of past sampling methods. Reduction or elimination of unnecessary early-season insecticide applications would save growers about $35 per acre. The reduction of unnecessary insecticide spray to curb the spread of Bd has additionally saved growers insecticide costs and reduced disruption to the orchard ecosystem. Proper use of ground covers, to increase soil health has benefits not measured in this study. Finally, this work clearly showed that early-season hemipteran pests may not cause significant damage because of plant compensation – therefore, pesticides are not always needed. We recommend early season insecticide treatment not be used unless insect numbers are higher than the plant can compensate for.

Reaction from Farmers and Ranchers

In 1998, the pistachio industry was hit hard by a fungal disease (Botryosphaeria dothidea [Bd]). During the past three years, most farmers have focused their attention on this pest and on methods to reduce its spread. The most important information from our past two years (SARE research) is that insect are poor vectors of this pathogen. This information has been presented at many grower symposia and has been accepted with a great deal of relief (some growers were applying three to four insecticide applications in a misguided effort to kill vectors of Bd. Our work also showed that while Bd spores are present on the ground covers, the understory plant growth does not increase Bd incidence, provided farmers adopt a good Bd sanitation program. Finally, trap crops were found to be most useful against the small bugs (e.g., Calocoris, Lygus). Our work showed that trap crops do arrest these insects from moving to the tree canopy (for a short time) and that their early season feeding does not increase Bd incidence.

We also have recommended that sweep net samples be taken of any ground vegetation to determine presence of migratory or resident hemipteran pests. This work, too, has been adopted, although we have not yet determined treatment levels.

Our work has been well received by growers and the pistachio industry. This is due, to a large part, to the personnel involved with the project. We worked on farms with forward-thinking farm managers, many of whom were members of the California Pistachio Commission Research Advisory Board. The Cooperative Extension personnel listed as cooperators were actively involved with this project and have an excellent reputation with the pistachio farm community. The relationship between personnel and clientele is often as important as the message being delivered.

This summary was prepared by the project coordinator for the 2000 reporting cycle.