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LEARNINGS FROM FIRE ANT BAITING

Here we look at the learnings pest managers can take from large-scale ant baiting programs – such as those tackling the red imported fire ant – and how they can be applied to smaller pest jobs. 

The red imported fire ant is one of the most researched pest ant species in the world. Baits are used as a key component in both government re ant control programs and commercial treatments and there has been significant effort in fine tuning both bait formulations and application techniques. What learnings can be taken from these re ant bait programs to improve ant baiting techniques on other species?

As with any ant treatment program, understanding the colony structure and their behaviours is key. How big can the colony get? Does the colony consist of more than one nest? Is it a monogyne (single queen) or polygyne (multi-queen) species? What are their food preferences and foraging behaviours?

The red imported re ant, Solenopsis invicta (pictured above), is somewhat unique in that it has both monogyne and polygyne variants. (The main imported fire ant incursion consists of both variants.) The monogyne variant can produce large single nests of up to 250,000 individuals. They produce alates which mate on the wing and there can be several flights each year. With queens able to disperse up to 500 metres, monogyne colonies can extend their range quite rapidly. Monogyne colonies will defend their territory very aggressively, which limits the number of nests in a given area.

In contrast, polygyne nests contain multiple queens and create new nests through budding – a queen and small group of workers will move away to a nearby site to start a new nest. There is no aggression between these nests and indeed there is often movement of workers between nests. As such, the nest density of polygyne colonies can be a lot greater than areas infested with monogyne colonies. Indeed, in many ways polygyne infestations can be considered as one ‘super colony’.

Baiting a single monogyne colony is relatively straightforward, with only the one queen to kill. As long as a reasonable amount of bait is provided (and they take and process the bait), colony control is assured. However, if you are treating an area of monogyne colonies it is important to ensure bait coverage over the entire area, as colonies will aggressively fight over resources and you need to ensure each colony takes enough bait back to the nest.

For polygyne colonies with multiple queens it can be more of a challenge to gain complete control. With multiple queens to kill, it is important to ensure sufficient bait is applied – quite often this may mean carrying out more than one treatment. In addition, although it is possible to eliminate ants from the treated area, there is a high likelihood of reinvasion from surrounding areas. This is why in the fire ant program they apply several applications a year and include a buffer zone beyond the known infested area. All the invasive ants species, including coastal brown ants and Argentine ants, have polygynous colonies so in many cases annual treatments may be required to keep them at bay, sometimes using a combination of baits and non-repellent sprays.

When choosing a bait for the pest ant in question, not only is it important to choose one that is attractive but the bait needs to be applied when the ant is actively foraging. Fire ants are generally only actively foraging when the air temperature reaches at least 20°C but are optimally active between 25-33°C. However, the soil temperature is a more accurate predictor of foraging activity, with ants foraging when the soil temperature is between 27-40°C. The effect of this behaviour is that baiting during winter is often less effective and during the summer baiting during the day when the ground temperatures are too hot is also likely to result in poor bait pick up, with the ants back in the nest.

However, the notion that bait could be applied during the day for the ants to pick up when the temperature cools in the evening can also deliver variable results. The actives used in bait products will have different levels of UV stability. For many baits the active can quickly degrade, making the bait ineffective. Fortunately, pyriproxyfen – the IGR used in both Distance Plus and Synergy Pro – is very stable in sunlight. That said, it is still best practice to apply bait when ants are actively foraging.

It is the incorporation of pyriproxyfen in Sumitomo ant baits that gives them their reliable performance under a wide range of conditions. Both Distance Plus and Synergy Pro are registered for use against re ants and a wide number of pest ants. Synergy Pro with its two different food granules (corn grit and protein) and combination of pyriproxyfen and hydramethylnon is generally the first choice in most situations due to is broad spectrum appeal and fast action, with colony control achieved in 1-2 weeks. Distance Plus only contains pyriproxyfen so consequently takes several weeks to deliver colony control. However, its safety profile means it is registered for use in sensitive accounts and cropping areas. Additionally, in areas where regular treatments are required, it provides a value for money option.

So in summary, successful baiting starts with correct species identification and understanding the colony structure (number of nests and number of queens in a colony). Inspection will determine the extent of the infestation and therefore the area of bait treatment. Application of sufficient quantities of a suitable bait, when the ants are actively foraging, then becomes the critical factor in success. The need for additional applications to complete eradication or as part of an ongoing management plan will then be determined by the level of infestation and species present.