Enhancement of Aphidophaga with Summer Intercrops in Pecan Orchards

James D. Dutcher, Entomology Department, Coastal Plain Experiment Station, University of Georgia, Tifton, Georgia, 31793-0748

    Aphids are particularly difficult to control in pecan orchards where broad spectrum insecticides are applied for control of nut pests, herbicides are applied to remove all vegetation from the orchard floor and producers are encouraged to maintain a foliage nitrogen content of 3% thus providing a lush diet for aphids in comparison to native trees. The result has been a very clean crop with respect to insect and disease damage to the nut and a concomitant destruction of natural enemies leading to resurgnece of secondary pest populations. The complex of aphids feeding on pecan foliage causes early defoliation and indirect loss of production by reducing the next seasons set of pistillate flowers. Poor aphid control for three consecutive seasons will stop nut production.
    Complex trophic interactions in the pecan canopy have been identified for phytophagous aphids, lepidoptera and mites. Conservation techniques have recently been devised for the aphidophaga. These include partitioning foraging behavior of red imported fire ant to prevent ants from tending aphids and removing predators from the tree. Even though pecan aphid populations are regulated by aphidophaga and entomopathogenic fungi - the aphids remain at a population size sufficient to produce enough honeydew to support the growth of sooty mold.
    Conservation techniques that increase the population levels of aphidophaga and entomopathogenic fiingi in the pecan orchard include: reducing the frequency of pesticide sprays; planting legumes as intercrops in the orchard to produce alternate prey aphids for aphidophaga; and, partitioning of the foraging behavior of the red imported fire ant with trunk sprays of insecticide to exclude ants from aphids and mealybugs in the tree yet retaining the ants on the orchard floor as predators of pecan weevil larvae. Implementation of the conservation techniques is often confounded by interactions with climate, secondary predators, and predator behavior. Combination of intercrops and removal of ants as secondary predators from the tree crown were found to enhance aphidophaga.
    The SRIPM project set out controlled field experiments in improved pecan orchards in Georgia and Alabama to determine the effects of intercrops and restricting the foraging by red imported on the abundance of aphids and aphidophagaous insects in pecan orchards. The climate was extreme during the two seasons of the experiments. lEgh ambient temperatures and drought in 1993 were followed by a midsummer flood in 1994 with continued heavy rainfall to the end of the season. In the first experiment, hairy indigo, hemp sesbania, hairy indigo and hemp sesbania mixture, and mowed sod were compared as warm-season intercrops with and without a chlorpyrifos trunk barrier to control red imported fire ant foraging. In trees with ants present, hemp sesbania intercrops where associated with lower overall blackmargined aphid and aphidophaga populations than in the other intercrop treatments. Ant barriers eliminated ant foraging in the tree crown and reduced blackmargined aphid populations on some sample date in mowed sod, hairy indigo and hemp sesbania intercrops. In the hairy indigo + hemp sesbania intercrop, there were more blackmargined aphids at the late season peak. Coccinellid populations were generally the same in all intercrop treatments. In the second experiment, hairy indigo and hemp sesbania sustained populations of the banded wing whitefly and not cowpea aphids during 1993. Ambient temperatures were too high for pecan aphid population growth and the whiteflies were associated with an increase in H. convergens. The whiteffies did not attract significant populations of C. septempuntata, O. v-nigrum, or H. axyridis to the intercrops. Olla v-nigrum populations will not usually reproduce in the field on whitefly populations alone. Red imported fire ant foraging patterns indicated that there were generally more ants in trees that were adjacent to an intercrop than in trees adjacent to mowed sod. Ants were more active during the mid-summer with reduced activity in the trees in the spring and fall. Blackmargined aphids returned in the spring of 1994 and the spring peak abundance was two to three times higher in trees without intercrops than in trees with intercrops. There were no discernible effects of the treatments on the abundance of ladybeetles in the trees. Blackmargined aphids were significantly higher in trees without intercrops and with ants excluded on the sample date before the flood. After the flood, continuous rainfall and a large immigration of the multicolored Asian ladybeetle, H. axyridis, reduced aphids in the trees to zero and ant exclusion did not have a significant effect on the abundance of eggs, larvae or adults in the trees. Overall, pecan aphid populations were influenced by the intercrop and ant exclusion treatments. The third orchard was divided into four large 1.35 ha (40 tree) and two were sown with a hemp sesbania summer intercrop and two were mowed sod. Pecan aphids and mites were monitored during the late summer. Fewer blackmargined aphids and pecan leaf scorch mites, the same number of yellow pecan aphids and more black pecan aphids were found in tree adjacent to the intercrops than in trees without intercrops in 1993. During a midsummer aphid outbreak, in 1994, blackmargined aphid abundance was higher in the mowed orchard plots than in the intercropped plots.
    Long term orchard management practices are effective in pecan orchards to stabilize the abundance of imported and indigenous natural enemies. The producer has a long term investment in the orchard and can implement conservation techniques over several years. Control of aphids in improved and seedling pecan orchards requires a combination of enhancement and conservation techniques to regulate aphids. The effects of intercropping and ant exclusion on the abundance of pecan aphids are evident in our results indicating that the combination of enhancement techniques may conserve aphidophaga and reduce pecan aphid populations. Enhancement techniques cause greater reductions in early season pecan aphid abundance than during the late season. The reduction in aphids due to exclusion of red imported fire ants from the tree crown is also more important in the midseason. The introduction of multicolored Asian ladybeetle to the pecan ecosystem may displace the indigenous ladybeetles. Larvae commonly are seen preying on the pupae of O. v-nigrum. The multicolored Asian ladybeetle is an improvement over the indigenous ladybeetles with respect to pecan aphid control and its ability to fend off attacks be the red imported fire ant. The development and implementation of an alternative to chemical control of pecan aphids will require greater monitoring and another level of decision making for producers. Fortunately, communication networks, identification of aphid resistant pecan cultivars and further research on host plant - aphid interactions win develop more technological support for the pecan production system.