Argentine Ants Must Be Suppressed

by Everett J. (Deke) Dietrick, President, Rincon-Vitova Insectaries

The exotic Argentine ant, Linepithema humile (formerly Iridomyrmex humilis (Mayr)) can quickly spread over large areas of farms and greenhouses. A rise in honeydew-secreting insect pests follows such invasions of Argentine ants. Good biological control is not possible unless honeydew-seeking ants are suppressed.It is well documented that large populations of Argentine ants can cause disruption of all the diverse sets of predators and parasites and antagonists involved in biological control. These ants eat beneficial insects and keep many beneficials away from plant-sucking prey, harvesting the insect honeydew for themselves.

When honeydew-secreting insects like aphids, whitefly, and soft scale flourish, they attract more beneficials. They even attract beneficials away from other crops and plants where there are no ants. Once they enter ant-infested plants, they are eaten, providing the protein to go with the carbohydrates in the insect honeydew. Such a balanced diet is so nourishing that the ants quickly produce multiple queens and more colonies.

The actual decimation of beneficial insect populations is most evident in those plants that are at the interface; in other words, where there are plants both with and without ants. That is where the battle of good and bad bugs is raging and the ants usually prevail. Ants may even transport certain aphids and other insects or store them in the nest for protection and future use to start new colonies after a crisis.Edward 0. Wilson, Harvard professor and expert on ants, says in his book Diversity of Life, "...the prevalence of ants must have something to do with their advanced colonial organization. A colony is a super organism, an assembly of workers so tightly knit around a mother queen as to act like a single, well-coordinated entity. A wasp or other solitary insect [a natural enemy] encountering a worker ant on its nest faces more than just another insect. It faces the worker and all her sisters, united by instinct to protect the queen, seize control of territory, and further the growth of the colony. Workers are little more than kamikazes, prepared--eager--to die in order to defend the next or gain control of a food source. Their deaths matter no more to the colony than the loss of hair or a claw tip to a solitary, animal.

"Again quoting from Wilson: "There is another way to look at an ant colony. Workers foraging around their nest are not merely insects searching for food. They are a living web cast out by the superorganism, ready to congeal over rich food finds or shrink back from the most formidable enemies. Superorganisms can control and dominate the ground and treetops in competition with ordinary, solitary organisms, and that is surely why ants live everywhere in such great numbers.

"Conventional pest management fights them with pesticides. Since most of the ants in a colony are beneath the ground surface in the nest working to protect the queen and tending the brood, the applications of pesticides has little direct effect on the colony. Management comes from destruction of all food sources above ground and from residues left that are toxic by contact. Ant colony reduction occurs mostly from starvation which forces reduction of the colony through cannibalism on the young to save the integrity of the colony.

Ants can cause both primary and secondary pest outbreaks. We recently saw tomato russet mite in the tomatoes in an organic vegetable greenhouse where Argentine ants were thriving on honeydew from aphids spreading along ant trails. A conventional grower would have sprayed for aphids, reducing the food supply to the ants and slowing down the whole process of ant-induced aphid damage, but might not have sought a root cause that would have prevented the need of pesticide in the first place.

Being an organic grower, biological control was desired and indeed a great deal of biological control activity was present. Though some of the aphid species did not have parasitoids, a parasitoid of green peach aphid, Aphidius matricariae, was present on the pepper plants. It was able to evade the ants, but it arrived too late and worked too slowly without the help of predators. Plenty of ladybugs came in, but their effectiveness was deterred by ants and their eggs and larvae offered more nutritious food for the ants. Releases of green lacewing to help the ladybugs and aphid parasitoids, only provided additional protein for the ants.

The Aphidius parasitoid eventually, temporarily eradicated green peach aphid and saved the peppers, after which a rise in hyperparasites of Aphidius brought the system into equilibrium. However, the ants immediately threw it off balance again. Where the ants had caused the secondary mite outbreak on the tomatoes, it was a pretty hopeless situation that could have been avoided.

The effects in citrus were studied many years ago. Where ants were excluded or removed, biological control was optimized. Where ants were allowed to interfere, all pests of citrus were observed to increase. When augmentation of natural enemies is an option, it is only cost-effective when ants are excluded. Biological control by natural enemies cannot be evaluated fairly in the presence of Argentine ants any more than it can in the presence of dust and pesticide drift.Argentine ants form new colonies rapidly wherever scouts mark out food sources. In spring and summer, there is a progressive increase. Cool weather slows ant activity and allows struggling populations of beneficials to destroy pests on many plants. When old crops that are left in the field are ant-infested, the decaying plant material, which is normally a useful habitat for beneficials, only serves this role for the benefit of ants. While wet or foggy weather slows ant activity, heavy winter rains will drive ants from rain-soaked fields into greenhouses, houses, and barns.

Argentine ants can be baited with several ant syrups that have poisons. When the bait is carried to the nest and fed to the brood and poisons the queen, the colony is destroyed. A new queen must be fed the royal food that transforms her into a queen. This takes time which can then be spent disrupting ant nests by irrigation, cultivation, and reduction of old crops, and also repeated trail disruption. Ants do not nest where growing plants or mulches cover the ground. They prefer sun-baked soil to nest under and may even clean the nest area of vegetation in order to warm their nests. But they also need water. Drip irrigation lines provide a perfect source of water and serve like highways for the movement of ants to form new colonies. Unplanted borders near irrigation ditches can be chronically infested with ants.

The lifecycleof the Argentine ant from UC IPM Pest Notes #12   June 1995

We believe limited populations of ants may perform a useful service by creating "pest spots" that attract and support the development of populations of beneficials. This beneficial phenomenon is taking place; however, it does not warrant that growers try to conserve ants. It is virtually impossible to eradicate ants and if growers will attack them on all fronts, there will always be plenty left to provide the service of attracting beneficials. It is much better if ant-induced "pest spots" occur in refuge covercrops and not in market crops. Ants can spread like wildfire. It is easier to control a fire when it starts and is still small. The fire runs out of fuel and stops. Once the fire is spreading, it is very difficult to control.

Quick-fix pesticides have disguised the role that ants play in biological control of agricultural crops, so ant research has not been focused on how to suppress ants at tolerable levels. When the role of ants is not recognized, growers trying to reduce pesticide use and transition to organic farming practices may end up throwing money away on commercial natural enemies. It is not sufficiently emphasized how essential ant management is for the successful use of biological control by natural enemies.Note

The argentine ant is now established throughout the southern states and California. Isolated infestations have been reported from Missouri, Illinois, Maryland, Oregon and Washington.

Characteristics of an Ideal Bait

Strategies for baiting social insects are somewhat different than baiting other pests. Foraging ants gather food and then take it back to the nest to feed other members of the colony (trophallaxis). Only a small percentage of the nest is foraging at any one time. If a bait acts too swiftly. foragers will die before they are able to poison the rest of the colony, and most of the colony will escape. New foragers see the old ones dying, avoid the bait and move the colony. A good ant bait, then, must show delayed action. One definition of delayed action is less than 15% mortality after 24 hour exposure and better than 89% mortality in 20 days (Knight and Rust 1991; Stringer et al. 1964). However, even a bait toxicant that does not meet these standards may still give relief from an infestation.

As well as being slow acting, effective doses of the toxicant cannot be repellent. Repellency is to some degree overcome by using an enticing bait matrix. If a colony has been starved by effective sanitation measures, baits are also accepted more readily. Non-repellency might be the most important bait characteristic because if the bait is not taken by foragers, nothing happens at all.Baits must also be readily transferred by trophallaxis. For worker exchange, the bait must be liquid or chewed into very small particles. The physical state of the food is not a problem with larvae, as they are able to eat either solids or regurgitated liquids.

A bait should show delayed action over at least a 10- to 100-fold dosage range. This requirement is necessary due to dilution of the bait during trophallaxis. Enough residual poison must remain after regurgitation to kill the foragers, and the amount delivered must be concentrated enough to do the job, even if it is spread among a number of larvae.

Finally, relatively large amounts of the bait must not be poisonous to non-target species. For instance, if a dog ate a whole bait station, nothing should happen (Stringer et al. 1964).

Reprinted from Common Sense Pest Control Quarterly page 7, Vol. XI, No 4, Fall 1995 by permission