Tuesday, May 6, 2014

Temperature: the key to fire ant baiting

soil thermometer
A simple soil thermometer can tell you the optimal times
of year and times of day for fire ant bait application.
Fire ant baits are wonderful tools for managing fire ants.  They are relatively inexpensive, require little labor to apply as broadcast treatments, and are safe for both applicators and the environment.  One of the biggest limitations of baits is that they cannot be used all year round. Instead applications must be timed to periods when fire ants are actively looking for food, foraging in ant worker lingo.

Many years ago a researcher at Florida State University, named Sanford Porter, spent an entire year (three times a day, once a week) monitoring fire ants coming to little bits of hot dog. Along the way he carefully monitored surface and below-ground soil temperatures, relative humidity, time of day, soil moisture, rainfall, and air temperature.  Porter found that by far the best predictor of fire ants foraging (and thus, when they are most likely to find and collect bait) was when the temperature of the soil at 2 cm (a little less than an inch-deep) was between 72 and 97 degrees F.

It makes sense that fire ants would be most sensitive to soil temperatures at this depth, as this is about how deep fire ants travel in their foraging tunnels, where they travel 90% of the time.  In Porter's study, fire ants nearly always found baits when the soil temperature was in the favored range.

This morning and afternoon I went outdoors and took the soil temperature in the lawn surrounding my office in Dallas, TX.  The temperatures at one inch averaged between 74 and 82 degrees, in morning and afternoon. This is the sweet spot for fire ants, and indicates that all day today would be a great time to use fire ant baits.

Typically we suggest fire ant bait applications in north Texas be limited to the months of May through September.  This ideal baiting time will vary from one location to another, but the soil temperature rule of thumb should be consistent.  If you're not sure when to apply fire ant baits, check the soil temperature with a metal temperature probe.

Daily temperature fluctuations

Besides time of year, soil temperature is also influenced by time of day. Right now, on the grounds surrounding my office, anytime during the day would be a good time to broadcast fire ant bait. But as any seasoned Texan will tell you, there's a mighty big difference in temperatures between May and July.  In July soil temperatures, even at one inch-depth, soar well over 100 degrees, effectively shutting down most fire ant foraging during the day.

The best time to apply fire ant bait during the summer months is late in the day, in the evening. Bait applied in the morning hours, even when soil temperatures are still favorable, will quickly be exposed to high temperatures and high UV intensity, both of which are likely to render bait less palatable to ants.  By applying bait late in the day, it will be available to fire ants during their most favored time for foraging, throughout the night.

For more information about baiting for fire ants, see our publications on Managing Imported Fire Ants in Urban Areas and Broadcast Baits for Fire Ant Control.

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