How Ticks Hide in Plain Sight From Predatory Fire Ants
By Leslie Mertz, Ph.D.
Predatory ants, particularly red imported fire ants (Solenopsis invicta), will mob and bite anything that represents a food item or a threat. So, when researchers noticed that the numbers of certain types of ticks were lower in areas where predatory ants lived, they assumed that the ants were preying on ticks. That assumption is wrong, according to new research reported in late August the journal Environmental Entomology, which shows that at least seven species of predatory ants, including the red imported fire ant, actually ignore them.
The predatory ant-tick relationship is important because it helps researchers get a better handle on ways to fight the looming threat from southern cattle fever ticks (Rhipicephalus [Boophilus] microplus), says the study’s lead author Allan Showler, Ph.D., research entomologist at the U.S. Department of Agriculture-Agricultural Research Service’s Knipling-Bushland U.S. Livestock Insects Research Laboratory in Kerrville, Texas. Cattle fever ticks spread bovine babesiosis, a deadly cattle disease that had been eradicated in the United States in the 1940s. But the southern cattle fever tick reared its head again in 2016 on the deep South Texas coastal plains, and several counties there remain in permanent and temporary quarantine zones today.
Because cattle fever ticks are quarantined pests, Showler can’t use them in the field, so he is doing the next best thing and investigating the lone star tick (Amblyomma americanum). The lone star tick doesn’t transmit bovine babesiosis, but, like cattle fever ticks, they are metastriate ixodids, meaning they are both members of the Ixodidae family (hard ticks) and have a flap that covers part of the reproductive opening.
Showler and his colleagues tested the ants in different habitats and by offering them a variety of baits either set out in open vials or dropped directly onto the ground near and on the ant nests. The results were the same: While fire ants would immediately go after other baits, including slices of hot dogs, house fly carcasses, and live moth caterpillars, they paid no attention to ticks whether they were eggs, larvae, nymphs, or adults. “It was as if the ticks weren’t even there. The ants didn’t pick up the ticks; they didn’t seem to recognize them as prey or as enemies,” he says.
The only way the researchers were able to entice fire ants was to puncture the body of a blood-engorged adult female tick. At that point, the ants finally noticed the tick, recognized it as a food item, and attacked it and hauled its internal organs and bodily fluids back to a nest hole.
When the researchers looked at another predatory ant species, red harvester ants (Pogonomyrmex barbatus), they saw something a bit different, Showler says: “The harvester ants recognized the tick eggs and replete [fed] adults only as foreign bodies, like they might see a pebble or a piece of plastic, so they dragged eggs and replete adults away and dumped them outside their nest clearing or at the very periphery of the clearing.” The harvester ants ignored tick larvae, nymphs, and unfed adults.
Red imported fire ant, Solenopsis invicta, responses to A. americanum egg masses, unfed nymphs and adults, and other baits placed on the colony mound, subtropical thorn scrub, Cameron Co., TX, 16–18 October 2017, six replicates.
|Baita||Attack||Taken to colony hole||Percentage replicatesd|
|Hot dog (1 cm3)||Immediatec||Yes||100|
|Dead house fly (1)||Immediatec||Yes||100|
|Living geometrid larva (1)||Immediatec||Yes||100|
|Tick egg mass (1 cm3)b||No||—||100|
|Engorged tick larvae (1)b||No||—||100|
|Unfed tick nymph (1)b||No||—||100|
|Engorged tick nymph (1)b||No||—||100|
|Unfed tick adult (1)b||No||—||100|
|Engorged tick adult (1)b||No||—||100|
|Engorged tick adult, wounded (1)||Yesb||Only internal organs and fluids||100|
Experiments showed that red imported fire ants (Solenopsis invicta) would immediately set upon a variety of baits, but they ignored all life stages of ticks: eggs, larvae, nymphs, and adults. The only ticks they attacked were engorged adult female ticks that had been wounded. aTick life stages were all alive. bAnts often touched the tick life stages but did not appear to recognize them as food items or as enemies. cImmediate, within 3 s; Yes, within 10 s. d100% of the noted ant responses were the same for each of the bait types. (Table originally published in Showler et al 2019, Environmental Entomology)
These experiments show that metastriate ticks are somehow able to hide in plain sight. Showler believes the metastriate lineage of the ticks may be involved. Metastriate ticks—the larvae, nymphs, and adults—have glands that run around the circumference of the body and exude an oily substance. Although the eggs might not release the oil, they may also get coated with the oil when they are laid. This oil is made up of numerous compounds, including one called squalene that is present in the trail pheromone of many ants. “So, what may be happening is that squalene or perhaps other compounds in the oil might mask the tick from appearing to be a food item or an enemy,” Showler says.
While this might explain why fire ants don’t attack ticks, it doesn’t resolve why tick numbers decrease as fire ant numbers increase. Here, Showler says the answer may lie in the tick’s small-animal hosts, such as bobwhite quail and squirrels. “A number of research papers have shown that fire ants drive out these hosts, so I think what’s going on is that the ticks have nowhere to go to develop through their first stages. And, as a result, their populations decline,” he says.
Through this study, Showler is building a broader picture of the distribution of metastriate ixodid ticks in southern Texas. Based on previous research, he already knew that the ticks were uncommon in the highly saline soils of regions swept by southerly winds from the Mexican llano—mainly as a result of predation by dense populations of mud flat fiddler crabs—and the new findings are revealing why ticks aren’t also as severely depressed in low-salinity, thorn-thicket regions where predatory ants are present. “This helps us learn why ixodid ticks are distributed in the way that they are, and that then helps us to determine where we should be sampling for all sorts of ixodid ticks, including southern cattle fever ticks,” he says.
He hopes to use this understanding of tick ecology and biology to find effective methods to control them. He is especially interested in dusts, including those laced with extracts of chrysanthemum and thyme, which he describes as lethal to ticks, and he is working on different ways to get the dust onto the ticks and their hosts. “When I was originally brought in to study ticks in 2013, I really didn’t want to do it. I was a crop entomologist (mostly sugarcane and cotton) and worked on desert lcocusts in Africa and Asia. I thought ticks were disgusting,” he says. “But, as I have gotten into the work, I changed 180 degrees. This is a really fascinating puzzle and I like research puzzles.”
Leslie Mertz, Ph.D., teaches summer field-biology courses, writes about science, and runs an educational insect-identification website, www.knowyourinsects.org. She resides in northern Michigan.