August Field Reports
Kern County BASIC Pilot Program
Earlier weekly reports during August will be added as they are edited.
August 27, 2001
Everett Dietrick's comments from August 27 concerning SFA, Southern Fire Ant, Solenopsis xyloni, McCook.
References to the biology of fire ant are from Taber, Stephen W., Fire Ants. Texas A&M University Press, College Station, mostly from Chapter 3 "The Red Imported Fire Ant (Solenopsis invicta)".
This native species of ant is widely distributed in all five of the cotton fields. It has been called the "cotton ant" having been described along with cotton insects by McCook (1879) However, in California, it is one of the sub-species sometimes called the "western fire ant" that is both a beneficial predator and a pest that interferes with biological control.
Whitefly and Aphid Populations in the Cotton are related to Southern Fire Ant
Observations, in cotton this year, reveal that the SFA attacks other predators under certain circumstances, and will affect the destruction of many competing natural enemies in their efforts to protect their honey-dew food source provided by aphids and whitefly populations. By protecting and nurturing the honey-dew secreting pests and allowing them to increase in numbers, the whitefly and aphids can become major pest problems. The damage occurs when the bolls open by covering the cotton fibers with sticky sugars upon which sooty mold fungus, Meliola comelliae Catt, grows and results in sticky black fibers. The benefits of SFA as a predator are far outweighed by the interference it causes to successful biological control of cotton insects, particularly the whitefly and aphids in untreated cotton.
Fire Ants Interfere with Biological Control
When the more mature worker scouts of SFA discover a good source of food (honey-dew) they recruit other workers to come to this source by laying down a pheromone from the food source to the nest. This "bucket brigade" exploits the aphid or whitefly colony very efficiently. The resulting increase of food stimulates the queen to lay more eggs and the colony grows in size. More ants grow more ants to harvest more honeydew until the "ant interference" to biological control of these pests is destroyed. The whitefly and aphid pests increase and the hot spot spreads throughout the field. As the hot spot spreads and attracts more general feeding natural enemies of aphids and whitefly into the fray where they to are killed by the increasing ant population. Other plant feeders, such as mites that are not themselves attractive to the SFA, often are protected along the trails of the bucket brigade and become victims of the battle of the bugs. Good biological control cannot co-exist with the intolerable high density of SFA. When the plantscape favors SFA colony expansion the diversity of the natural enemy complex is destroyed and biological control fails. Since the SFA forages year round it interferes with biological control by the successful way it exploits the whitefly and aphid food source throughout the plantscape protecting them by destroying their natural enemies.
More is known about the red imported fire ants, RIFA, due to the millions of dollars and more than 30 years of eradication money spent on research which has resulted in failure.. However, the native species SFA's behavior is similar to RIFA in so many ways, that one can infer that SFA biology is only slightly less aggressive and probably has more native biological controls than the RIFA. Since the RIFA has recently extended its range into California, some best educated guesses for future management strategies for RIFA control are in order and can be learned from managing SFA. In any case, the evidence shows the RIFA out-competes SFA and eventually displaces it everywhere they come into contact.
Fire Ant Reproduction
No one has ever observed the complete mating process of any fire ants. The reason is that usually, perhaps always, it takes place on the wing and high above the ground, probably at heights of about 1000 feet. Airplane collected samples have gathered copulating pairs at a thousand feet. Flying males leave the nest slightly before the females and form a layer in the sky that the females must fly through which increases the likelihood of finding a mate. The females mate only once. The mated female, which is now a queen, lands after mating, sheds her wings and begins digging a nest to lay her eggs. This first brood of offspring are called minims. These workers, about eighteen in number are smaller than all other workers of later broods and differ in behavior. They all provide the food that helps the queen make the colony grow.
Colony size increases through nuptial flights of sexuals produced by the colony at any time of year. Prodigious reproductive ability and long life (several years) of the single queen per colony lies behind the ability of the fire ants to increase in numbers in so many agricultural crops within one seasons growth and repeat season after season.
Fire Ant Diet
SFA are presently reported to feed on almonds and other nuts during harvest time, steal planted seeds, and sting severely to the point of killing small animals and birds in the nest as they are hatching. However, they show some preference for protein foods to plant products by foraging on insects and even the germ part within the seeds. Artificial diets worked better when insects were substituted for the chemically pure nutrients in the diet.
All colony members can eat liquids, but only a selected group, the oldest larvae, can process solid foods. Liquids stored in the returning forager's crop are partly digested internally before being regurgitated to nest mates. There is a filter near the mouth of foragers that screens out solids about the size of a bacteria, 0.88 micrometers. Solids screened out along with those too large to enter the digestive tract accumulate in a pocket resulting in a pellet, which is fed to the oldest larvae, which do not filter their food and are the only group that can digest solid food. Food becomes equally distributed through a begging behavior until all of their crops are equally full.
The SFA attacks other insects for food and is known as being beneficial to cotton by attacking worms and boll weevils in cotton and worms in other crops, such as sugarcane and soybeans. However, when practicing biological control, SFA becomes a serious pest. The problem is that it destroys the biodiversity of the natural enemy complex by competing and attacking beneficial insects, particularly ladybug and green lacewing larvae. This ant can become the only predator of certain pests while interfering with biological control of other potential pests making them into major pests. The increasing dependence upon conventional pesticides in the southern San Joaquin Valley may be caused by the increased presence of the SFA.
Managing Habitat in Cotton Fields to Suppress Fire Ants
The increased use of herbicides and drip irrigation has provided the open exposed space that may increase the success of the establishment of new colonies from the nuptial flights. When the established large colonies expand and new colonies succeed in increasing the percentage of successful nest establishment from the one success in 1,000 mated females estimated for the RIFA to the higher success ratio, the SFA interference increases.
The relative success of colonization of the SFA in all five of the cotton fields we are observing this season has become an observable important factor in biological control of cotton pests, in particular the whitefly and to a lesser extent the cotton aphid. The evidence is accumulating that SFA management may turn out to be the key factor in biological control of cotton pests and ecologically based pest management in other crops, such as alfalfa hay, vineyards, citrus, corn, stone fruits, vegetables and field crops when grown organically or without pesticides.
SFA is a native ant species whose populations are widely distributed in all of the cotton fields. They prefer nesting in open exposed places, often forming loose mounds or numerous scattered craters from which they emerge day and night except during summer high temperature. They retreat to the protection of the nest in mid day, coming out about 1600 and foraging intensely through the night until cool temperatures slow their activity. The nuptial flights establish new colonies within the cultivated cotton fields that expand the interference to biological control throughout the field as each new nest grows more and more foraging workers.
Learning from Argentine Ant Interference with Citrus Biological Control
The symbiotic relationship between Argentine ants and citrus pests has been well documented during the transition from conventional chemical control to IPM programs that implement biological control by protecting and nurturing natural biological control such as releasing the California red scale parasites. Early findings in the research and demonstration phases of this project proved that ant interference was the limiting the effectiveness of the biological control processes. Where conventional chemical control was practiced, the ants were routinely being suppressed by sprays for other pests. When pesticide reduction occurred to reduce chemical interference to parasites and predators, the ants became the problem and it became necessary to directly suppress ant populations, reducing their numbers and nesting sites to prevent ant interference to biological control from raising the density of all citrus pests.
Cultural Modifications of the Fire Ant Nesting Areas are Needed
It appears from this years observations of the Southern Fire Ant's, SFA, behavior in cotton, that its symbiotic relationship with the whitefly and cotton aphid may interfere with biological control processes sufficiently to make it necessary to directly suppress SFA nesting sites. We want to find ways to enhance the biological control of the SFA through cultural modifications of the areas where they successfully establish their nests.