California citrus growers looking to protect biological control methods for critical pests should understand how a troublesome ant species can shipwreck the best of intentions.

The Argentine ant is common throughout much of the state's citrus growing region, particularly the South Coast and western Riverside County region. Because this is also where the Asian citrus psyllid is established, controlling the Argentine ant is critical in fostering beneficial pest populations that can help aid in psyllid control and the control of other phloem-feeding pests that excrete a sugary waste known as honey dew. These include mealybugs, aphids, whiteflies, and soft scales, according to Mark Hoddle, an entomologist at the University of California, Riverside.

Because of their ability to reproduce and benefit from these sap-sucking pests, Hoddle says the Argentine ant can quickly build populations that can overwhelm other pests in the area. While they do not cause direct plant damage, their numbers and aggressive nature are enough to beat back beneficial insects to insignificant levels.

For reasons not entirely understood, Hoddle says the Argentine ant is not an issue to growers in California's Central Valley region, home to much of the state's fresh orange production. Where it is an issue – namely southern California – it can wreak havoc on the best of IPM control intentions.

The delisting of chlorpyrifos products will change how citrus growers control the ant as those are currently the only approved insecticide available for Argentine ant control, he said.

What we do know

Argentine ants have a limited range from their respective colonies. Because there are multiple queens and multiple connected nests, they can build populations quickly. Nests are large and the insect feeds primarily on the sugars created by phloem-feeding pests.

Their aggressive nature rapidly excludes native ants from resources and have been implicated in reducing horned lizard populations. This is troubling as the horned lizard feeds on native ants and will not feed on the Argentine ant.

In exchange for this ready-made food source, the ants are highly protective of honeydew excreting pests. They are also known to drive other pests into new areas of the grove. As a side benefit, the Argentine ant can reduce the risk of fungal infections in citrus groves.

Argentine ant populations increase their activity in the summer across southern California, generally peaking from August through October. Their lifecycle is relatively long, and they can live for about a year. The average developmental lifecycle from egg to adult is about 74 days, Hoddle says.

New control methods

Hoddle is working with a team of researchers to help control Argentine ant populations through development of hydrogel technology that can be employed instead of expensive bait stations. The new technology appears to show promise as it looks to be more cost effective, easily spread throughout the grove, and is biodegradable.

Early indications suggest the tiny biodegradable beads, filled with sugar, water and an insecticide, attract foraging ants without attracting pollinators, such as honeybees.

Hoddle is also studying sensor technology. He believes the Internet of Things (IoT) has the potential to revolutionize ant monitoring over the old way of sitting in an orchard, manually counting ants as they ascend and descend a tree trunk.

By placing sensors with GPS tagging technology onto irrigation pipes laid across the soil, Hoddle hopes to develop a statistical prediction of how many ants climb trees in search of honeydew.

Hoddle calls these irrigation pipes "superhighways" for ants as they have figured how to use these to efficiently move throughout the grove.

"We've given them a very smooth surface to run over with our irrigation pipe, which enables them to move very quickly," he said.

One way to slow the ants along these "superhighways" is to effectively use mulch. Studies suggest three inches of mulch can greatly slow the ant's movement while providing soil health and other benefits to the orchard.

Outsmarting ants

Broad-spectrum insecticide barriers applied to citrus orchards may kill foraging ant workers, but they do not kill queens in the subterranean nests. Over time the ants figured out how to use bridges to circumvent chlorpyrifos residues.

Chlorpyrifos has been delisted in California and can no longer be used as an insecticide after the end of the year.

Broad spectrum pesticides are also non-discriminatory and can kill beneficial insects, Hoddle says. This is one of the reasons he is working on different types of bait applications – the hydrogel beads – to control ant populations.

To date the hydrogel beads show promise for their effectiveness and cost, but more research is needed to verify their efficacy.