Research from Japan’s University of Kyoto suggests that a virus that infects the ants could be the key to understanding – and manipulating – the feeding behaviour of the red imported fire ant. (Photo credit: Kyoto University/Yang)

A new study, which has revealed how a viral infection can cause red imported fire ants (RIFA) to change their dietary and feeding behaviour, suggests it may necessary to revaluate the current methods by which the ants are controlled.

Native to South America, fire ants (Solenopsis invicta) are imported pests with a fierce sting that pose a threat to agriculture and tourism. The Australian government spends $38 million a year controlling the pest via its National Red Imported Fire Ant Eradication Program.

Fire ants are known to feed on proteins and love oily foods. As such, baits often consist of corn grit soaked in a mixture of soybean oil and an insecticide or insect growth regulator. As with any baiting program, success is dependent on the ants finding the bait attractive and taking a sufficient quantity back to the nest to kill the colony.

The new study from Kyoto University published in Scientific Reports looked at the way fire ants responded to a pathogen, Solenopsis invicta virus 1 (SINV-1), a virus that only infects ant species in the genus Solenopsis.

The researchers wanted to test if fire ants, as social insects, responded in the same way as non-social insects. Non-social insects have evolved a variety of mechanical, chemical, and behavioural defensive responses to pathogens, such as reduced foraging behaviour and dietary changes.

The Kyoto team separated the fire ants into two groups: ants infected with SINV-1 and uninfected ants, as the control group. The team then monitored the feeding habits of each group.

The study showed the virus affected both feeding frequency and food preference of the infected ants, with two key findings: firstly, the infected group saw significantly fewer worker ants foraging for food; secondly, the infected group significantly preferred carbohydrate food sources (diluted honey or potato chips) over high lipid or protein content food (tuna and peanut butter).

The results have implications for current control methods. “These findings also suggest that viral prevalence in the field may potentially influence the efficacy of conventional chemical control by low-toxic baits,” said co-author DeWayne Shoemaker, from the University of Tennessee.

Traditional oil-based baits may have reduced efficacy with colonies affected by the virus. The ants are not foraging in great numbers and worse still, are seeking an entirely different food source. If the fire ants do eat the protein bait, then there is a concern the ants may not ingest a sufficient dose of bait for successful control. Indeed, this may explain why there is a higher prevalence of colonies with the virus in areas of regular baiting – more of the uninfected colonies have been eliminated.

Not only does the behaviour of infected ants impact the performance of control products, it can also impact monitoring methods. Protein lures, typically sausages, are used to monitor fire ant activity. However, based on this current finding, the absence of fire ants from baiting stations may not necessarily be an indication that RIFA are not present – it could simply mean they are under the e ect of this naturally occurring virus and their feeding habits have changed.

“We are now considering how to integrate the virus into existing chemical control schemes,” study co-author Chin-Cheng Scotty Yang, of Kyoto University explained.

It is not unreasonable to assume that similar viruses may impact the feeding preferences of other ant species – it could be another reason for the variation in feeding preferences observed in the field and why a bait may work well in one situation but not another.

Further reading: Hung-Wei Hsu, Ming-Chung Chiu, DeWayne Shoemaker, Chin-Cheng Scotty Yang. Viral infections in fire ants lead to reduced foraging activity and dietary changes. Scientific Reports, 2018; 8 (1).

Reworked extract from: ‘Virus may help combat fire ants, but caution is needed’. Science Daily, 11 September 2018.