Pest Management and Orchard Floor Management Strategies to Reduce Pesticide and Nitrogen Inputs

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.