Pest Management and Orchard Floor Management Strategies to Reduce Pesticide and Nitrogen Inputs
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.
Annual legume ground covers were evaluated in pecan (Carya illinoinensis [Wangenheim] K. Koch) to supply nitrogen and increase beneficial arthropods. Releases of Chrysopa rufilabris (Burmeister) (Neuroptera: Chrysopidae) were evaluated as a biological control for late-season pecan aphids.
Treatments were 5 ha of a ‘Dixie’ crimson clover (Trifolium incarnatum L.) and hairy vetch (Vicia villosa Roth) mixture and 5 ha of grass sod. Nitrogen was applied at 0-168 kg.ha-1 in 56 kg increments to grass plots but no nitrogen was applied to the legume plots.
The most abundant beneficial arthropods sampled in the legumes were spiders, lady beetles, green lacewings and nabids, respectively. Legume ground covers did not increase the densities of beneficial species in the pecan canopies. The most abundant beneficial arthropods in the pecan canopies were spiders, green lacewings, brown lacewings and lady beetles, respectively. Predominant lady beetle species in the legumes were Hippodamia convergens (Guerin-Meneville), Coleomegilla maculata lengi (Mulsant) and Coccinella septempunctata (L.) (Coleoptera: Coccinellidae) while the most abundant species in the pecan canopies were Olla v-nigrum (Mulsant), Cycloneda munda (Say) and Hippodamia convergens.
Releases of Chrysopa rufilabris were not effective as a biological control of pecan aphids. Release of Trichogramma sp. to control pecan nut casebearer (Acrobasis nuxvorella) and hickory shuckworm (Acrobasis nuxvorella) was ineffective. The legume mixture supplied over 100 kg.ha-1 N to the pecan trees. Legumes significantly increased soil nitrate concentration during the fall compared to soil from grass plots.
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. However, beneficials in the pecan canopies were similar using either a legume or non-legume ground cover. Aphids feeding in the pecan canopies were usually lower using a legume ground cover.
Release of green lacewings for mid- and late-season aphid control was detrimental to lacewing populations. 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.
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. Legume ground cover may eliminate one to two pesticide applications, plus reduce mowing costs and 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.