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.