Final Report for LS91-036
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The purpose of this study is to develop and test a pest management and orchard floor management system for use by pecan growers. The system utilizes winter legumes interplanted in the orchard to produce and manage native beneficial insect predators and parasites for early-season aphid control. The system also utilizes release of commercially available predators and parasites for mid- and late-season aphid control and control of lepidopterous pests. Legumes are managed to supply the pecan nitrogen requirement.
Beneficial arthropods were more abundant in orchards with legume ground covers compared to those orchards with grass ground covers. These beneficials were attracted into the orchards because of the large aphid populations feeding on the legumes. The high beneficial populations controlled aphids in the pecan canopies, eliminating the need for pesticide applications to control aphids. Benefits of releasing of green lacewings for mid- and late-season aphid control was unclear. Although green lacewing egg laying and pupae were increased by supplemental releases, parasitism of the green lacewing eggs and pupae was increased, thus populations of green lacewings were similar or lower than in orchards without supplemental releases. However, in both cases aphids did not increase to injurious levels because of high lady beetle populations and other aphid predators. Trichogramma releases to control pecan nut casebearer and hickory shuckworm were not successful. Although populations of these two lepidopterous pests were reduced, their damage was above acceptable levels. Legume ground covers produced abundant nitrogen which satisfied the nitrogen requirement of the pecans. Our results indicated that legumes supplied the equivalent of 130 to 165 pounds of nitrogen/acre to the pecan trees. An additional benefit of annual legume ground covers was a reduction in the number of times the orchard required mowing. Because the legumes formed a dense mulch when they senesced, orchards were only mowed twice compared to five to six times with grass ground covers. We estimate that legume ground could eliminate one to two pesticide applications, reduce mowing costs plus supply the nitrogen requirement; however, costs would be added for seed bed preparation and seed, with a net reduction in input costs of about $20 to $40/acre.
To demonstrate the advantages of utilizing winter cover crops of legumes in commercial pecan orchards to produce beneficial insects for the control of aphids and other pests on pecan trees.
To test and evaluate the use of certain commercially available beneficial insects for control of aphids and lepidopterous pests of pecan trees.
Evaluate selected legumes to reduce commercial nitrogen inputs for pecan production.
Objective 1. Legume plots in Oklahoma include two sites with a mixed stand of crimson clover plus hairy vetch in native pecan groves. Check plots were grass ground covers (fescue at one site and bermudagrass at the other site) with commercial pecan management techniques. In Georgia the two sites were cultivar orchards with crimson clover alternated in the row middles with hairy vetch. Grass check plots were commercially managed.
Insect populations were monitored at seven to 14 day intervals on the legume ground covers using sweep nets, and pecan canopies were sampled with a vacuum device. Insects of interest were identified and counted. Aphid (black margined, yellow and black aphids) and green lacewing egg populations in pecan canopies were counted on ten compound leaves/tree on ten trees/plot. Beneficial predators monitored included lady beetles (five species), green lacewings (two species), brown lacewings, nabids, assassin bugs (three species), syrphids and spiders. Aphids on the legumes (pea aphid, cowpea aphid and blue alfalfa aphid) were counted from sweep net samples. In depth discussions of each predator and pest along with their population dynamics is not practical in this report; therefore, only certain data will be highlighted. More in-depth information is available in the attached manuscripts, and additional manuscripts are being prepared.
Data from this study indicated that legume ground covers were successful in increasing certain beneficial arthropods. In particular, lady beetle populations on the legumes were abundant (Fig. 1) and were increased in the tree canopies compared to orchards with grass ground covers. Lady beetle populations on the legumes were usually closely associated with aphid populations (food source) on the legumes; however, in some instances they appeared to associated with the tarnish plant bug nymph population. Tarnish bugs are not harmful to pecan trees, but can serve as a food source for certain beneficial arthropods. The abundance of each lady beetle species in the legumes varied with orchard. For instance, at one site Coleomegilla maculata lengi was the most abundant lady beetle (50% of the populations) followed by Hippodamia convergens (26%), Coccinella septempunctata (14%) and lastly Cycloneda munda (9%). At another orchard Hippodamia convergens (43%) was the most abundant species followed by Coccinella septempunctata (23%), Coleomegilla maculata lengi (22%), with small populations of Cycloneda munda (10%) and Olla v-nigrum (2%).
Lady beetle species found in the pecan canopies differed from those in the legumes. In orchard one above, 57% of the lady beetles in the pecan canopy were Cycloneda munda compared to 9% Cycloneda munda found in the legumes. Olla v-nigrum and Hippodamia convergens followed as the second and third most abundant species found in the pecan canopies at both sites. Even though a large percentage of the lady beetles were Coleomegilla maculata lengi and Coccinella septempunctata in the legumes, they do not appear to play as important a role in aphid control in the pecan canopies. Cycloneda munda, Olla v-nigrum, and Hippodamia convergens were the predominate lady beetle species in pecan canopies.
Green lacewing populations on the crimson clover/hairy vetch increased until 27 May then declined when the legumes senescesed and legume aphid populations dropped (Fig. 2). The decline in green lacewings on the legumes occurred at the same time lacewing populations increased in the pecan canopies, suggesting a migration from the legumes to the pecans in search of food. There were two peaks in the green lacewing populations (Fig. 2), the first associated with migration from the legumes and the second associated with a late season increase in yellow aphids in the pecans (Fig. 3).
There were two distinct peaks in the aphid populations (black margined aphid + yellow aphid) on the pecans (Fig. 3). The first coincided with a peak in lady beetle populations in the canopy, and the second with increased green lacewing populations. The first migration of green lacewings from the legumes occurred as aphid populations in the pecan canopy were declining (Fig. 2). If weather conditions favored aphid increases, then migration of lacewings from the legumes would be essential for control. During both the first and second peaks in aphid populations, plots with crimson clover/hairy vetch had fewer aphids than control plots. The rapid decrease in aphid population during spring can be attributed to increased activity of lady beetles and other predators. Similar results were obtained from the other sites. For example, pecan aphids in Georgia averaged 6.6 aphids/compound leaf using legume ground covers and 10.3 aphids/compound leaf with grass ground covers. These data suggest that legume ground covers can be used to attract beneficial insects which control aphids and suppress other pecan pests.
Our data indicated that yield from orchards with legume or and grass ground covers was similar. Therefore, low-input orchard management using a legume ground cover would reduce input costs without causing a yield reduction.
Objective 2. Green lacewings were reared at Byron, Ga and either used in studies at Georgia or were shipped to Oklahoma for use. Lacewings were released in plots containing crimson clover/hairy vetch during 15 July, 1 August, or 15 August to supplement native beneficials in controlling aphids. A total of 1500 adult lacewings were released per acre.
Lacewings immediately entered the pecan canopy upon release. Subsequent sampling of pecan canopies indicated that supplemental releases had little impact on the number of adults (Fig. 2), larvae or eggs (data not shown) in pecan canopies. The ineffectiveness of supplemental releases was attributed to low populations of aphids at the time of release causing migration out of the orchard in some cases, and in others high parasitism of lacewing eggs and pupae. The high degree of parasitism is particularly problematic. It suggests that if lacewings are maintained at high levels in the orchard with supplemental releases, then their parasitoids increase to levels that actually result in fewer lacewings. Our data suggests that supplemental releases to maintain high lacewing populations to prevent an aphid build-up are not practical.
Trichogramma are a parasitiod of lepidoptera eggs and possibly other pests. In 1992 we evaluated a commercially available Trichogramma which had been isolated from walnut bud moth. In addition, we were successful in isolating native Trichogramma from hickory shuckworm eggs on pecan. The Trichogramma isolated from pecan were evaluated during 1993.
In Oklahoma, Trichogramma were released in the crimson clover/hairy vetch plots when the first pecan nut casebearer eggs were detected (June 5 at both sites)(50,000 Trichogramma per acre). Parasitized eggs on construction paper were hung at the ends of branches. There were five release sites per acre. The day following release we confirmed Trichogramma emergence. No loss of the parasitized eggs was observed.
At one site plots with grass ground covers were either sprayed with chlorpyrifos (June 11) or not sprayed. At the other site casebearer egg laying was observed on only 1% of the fruit clusters, and no pesticides were applied. On June 30, twenty fruit clusters were counted on twenty trees per treatment at each site to determine casebearer damage.
Eighteen percent of the fruit clusters in unsprayed trees were damaged by casebearer compared to 17% damage using Trichogramma, and 0.5% using chlorpyrifos at the first site. Damage was less than 2% in both Trichogramma treated and untreated plots at the second site. These results indicate that Trichogramma did not satisfactorily control pecan nut casebearer.
In Georgia, Trichogramma isolated from hickory shuckworm eggs were released for during July and August for hickory shuckworm control. Results indicated that Trichogramma were ineffective in controlling shuckworm, even at very high release rates.
Objective 3. Nitrogen rates of 0 to 200 lb. N/ acre in 50 lb. intervals were applied to grass plots to determine the nitrogen value of the legumes to the pecan orchard. Pecan leaf samples were collected in July from nitrogen and legume plots and analyzed using the Kjeldahl procedure. Regression equations were calculated to determine the relationship between leaf nitrogen concentration with nitrogen application rate. The apparent nitrogen supplied to the pecan trees by the legumes was the calculated based on the regression equations.
Crimson clover plus hairy vetch supplied the equivalent of 130 to 165 lbs. of nitrogen per acre depending on the year and site. Pecan leaf nitrogen values in the crimson clover/hairy vetch plots were 2.5% to 2.8% which are within the normal nitrogen concentration range. Our data indicate that a well-managed crimson clover/hairy vetch ground cover is capable of supplying to total nitrogen requirement for pecan production.
Educational & Outreach Activities
A field day was held on 2 June 1992 at Bob Knight’s orchard which featured the work on this project. There were about 150 pecan growers at this meeting.
The Creek County Farmers’ organization toured selected farms 21 May 1992. Bob Knights’ orchard and the work on this project was highlighted on this tour. About 40 farmers participated in the tour.
A field day at Knight orchard during August 1993 discussed this research along with other research projects at this site. There were about 55 in attendance.
A Tulsa TV station featured this work on the environmental reporting segment of their news program. The report included footage of the plots and interviews with Bob Knight, grower, Ray Eikenbary, entomologist and Mike Smith, horticulturist.
Five presentations to grower organizations were made concerning this work. The first was to the Oklahoma Pecan Growers’ organization in 1992 at Texoma Lodge near Durant. There were about 250 growers at the meeting. The second was to the Northern Nut Growers’ Association in 1992 at Michigan State University, East Lansing. There were about 175 growers in attendance. The third presentation was to the Oklahoma Pecan Growers’ Association in 1993 at Shangri-La with about 230 in attendance. The fourth was at the Northern Nut Growers’ meeting in Pittsburgh, Ks with about 120 in attendance. The fifth was a grower group in Georgia with about 20 in attendance. I am also scheduled to present this work at the Horticulture Industry Show during January 1994, and at the Western Pecan Conference during March 1994.
This work will be feature on Sun-Up, an OSU produced TV program during January.
A popular article featured this work in the Oklahoma Farmer-Stockman, November, 1993 (see attached article).
This work has been presented at two professional meetings (see below).
Rice, N.R. and M.W. Smith. 1992. Evaluation of low-input pecan orchard floor management systems. HortScience. 27:639.
Rice, N.R., M.W. Smith, R.D. Eikenbary, W.L. Tedders, B.W. Wood, G.G. Taylor, B.S. Landgraf, G.E. Barlow. 1993. Evaluation of low-input pecan orchard floor management systems. HortScience 28:478.
Two manuscripts have been submitted for publication (see attached), and additional manuscripts are being prepared.
This project addresses two key issues of concern today. Those issues are the utilization of pesticides and commercial nitrogen fertilizers. About 10,000,000 pounds of insecticides are applied annually to pecan orchards. This increases pesticide loading in the environment, and is a major expense for the growers which limits profitability. The pest management methods would substantially reduce the dependence on pesticides to produce high-quality crops, and reduce grower expense when implemented. Therefore, results of this study when implemented by growers would reduce pesticide usage benefiting the environment, water quality, pesticide effects on non-target organisms plus increase profitability since these management practices are less expensive than conventional management.
Nitrate pollution of the ground water and surface water has been identified as a national concern. Non-point nitrate sources remain a large contributor to low quality water. Therefore, practices which reduce the likelihood of nitrate leaching or runoff will reduce the contribution from non-point sources. Additionally, the large increase in commercial nitrogen costs in recent years have contributed to a reduction in grower profitability. Utilization of legumes as ground covers to supply nitrogen for pecan management can reduce the likelihood of nitrate loss from orchards and reduce expenses compared to commercial fertilizers when all benefits of the legumes are considered. Commercial nitrogen applications are normally applied during the spring when rainfall is greatest. High rainfall increases nitrogen loss by leaching and surface runoff which leads to water pollution. Release of nitrogen from legumes is gradual as the legume residues decompose, reducing the probability of leaching. Additionally, when growth of the cool season legumes begin in the fall available nitrogen in the soil is absorbed by the legume preventing nitrate leaching during the winter.
This research has demonstrated that legumes can be utilized to eliminate certain pesticide applications and commercial nitrogen applications. Several native orchards include beef cattle production. The effects of different levels of grazing on arthropod habitat, and nitrogen cycling with livestock/legumes/pecans requires investigation to develop low-input profitable management systems for this type of operation.
Our data indicates that the use of legumes as a ground cover to eliminate certain pesticide applications and commercial applications did not affect yield. Therefore, its economic benefit must be evaluated based on cost of commonly used management practices versus the low-input alternatives. Using annual legumes as the ground cover resulted in a net reduction in costs of $20 to $40/acre.
Supplemental releases of green lacewing for mid- and late-season aphid control and Trichogramma for pecan nut casebearer and hickory shuckworm control were ineffective.
Changes in Practice
We have developed a low input management strategy for pecans which was not currently known. Information is being disseminated to grower groups as rapidly as possible. Several growers have adapted this management concept; however, we are unable to estimate the number of farms using this approach.
We are currently recommending crimson clover/hairy vetch ground covers in pecan orchards. This reduces pesticide applications for aphids, and the need for supplemental nitrogen applications in many instances.
Based on this and other research, we do not recommend releasing green lacewing or Trichogramma for biological control of pecan pests.
Areas needing additional study
A critical need in pecan management is to determine the maximum crop a tree can support to produce good quality nuts with annual yields, and develop a method to predict the crop early in the season. This would be important horticulturally and would support pest control decisions and marketing plans.
Additional information concerning economically damaging populations of certain pests on pecan would be useful to improve management decisions. Although there are some guidelines, many of these have been developed without experimental support or under a rigid management program which may not be applicable to low input management systems.
Monitoring systems have been developed for pecan weevil and pecan nut casebearer, and a model has been developed to predict egg laying of the pecan nut casebearer. Research is in progress to develop a pheromone for pecan weevil and test a pheromone for hickory shuck worm. Development of pheromones for all major pecan pests together with damage associated with trap catches would improve pest management, and reduce unnecessary pesticide applications.
Additional work on host plant, pest, predator or parasite interactions would increase our basic understanding of this relationship, and lead to improved management practices.
Critical leaf nitrogen concentrations in pecan leaves have been determined for maximum production, and are used to guide fertility programs. However, there is little research to support efficient scheduling of nitrogen applications to prevent leaching and surface runoff plus improve nitrogen use efficiency. Improved scheduling of commercial nitrogen applications, the use of legumes as a nitrogen source, or the use of legumes plus a commercial nitrogen source could improve production efficiency and reduce nitrate pollution.