Pestology Blog

The Pharoah ant, Monomorium pharaonis (L.), is a notoriously difficult pest ant to manage. These tiny ants can thrive indoors, preferring to nest in inaccessible areas such as wall voids and in attics where it can be difficult to locate and target the reproductives. The Pharoah ant’s tiny size and ability to quickly spread throughout a structure makes this species a major pest in any residential or commercial building. This ant is even capable of mechanically transmitting over a dozen pathogens such as Salmonella spp. and Staphylococcus spp., making it a serious public health threat in hospitals.  

Pharoah ant colonies consist of multiple queens, sometimes hundreds, distributed across dozens of satellite nests, called daughter nests, that are formed through a reproductive process called budding. Budding is the real key to this pest ant’s success and what makes it so difficult to manage, as this process allows this species to produce queens and new colonies on demand. Budding can be triggered by a number of factors including competition from other ants, a lack of resources, or even a chemical disturbance such as the application of a highly repellent insecticide.  

One challenge that professional pest control operators face is that clients often try to manage infestations themselves using over-the-counter products that typically contain highly repellent active ingredients. Previous studies have shown that these chemical disturbances can lead to budding in Pharoah ants, but these studies were conducted using similar sized sub-colonies. In the field, Pharoah ant colonies can vary in size from anywhere from 5 to 10,000 workers. Since previous studies only looked at exposure across standardized colonies, they didn’t address how chemical disturbances could, if at all, impact colonies of different sizes. 

To address this question, entomologists from the University of California Riverside exposed different sized Pharaoh ant subcolonies to three OTC aerosol products at three different application rates each, and monitored their effects over time. The subcolonies consisted of 100 workers, 10 mg of mixed brood stage, and 1 queen; 200 workers, 25 mg of mixed brood stage, and 2 queens; 400 workers, 50 mg of mixed brood stage, and 3 queens; and 800 workers, 100 mg of mixed brood stage, and 4 queens. The aerosols used were Zevo Crawling Insect Killer, Raid Ant & Roach Killer 26, and Black Flag Dry Ant and Roach Killer. The three concentrations tested for each product were 1, 2, and 3 seconds of aerosol spray.  

Fig. 1. From this paper. The experimental setup used in this study. The polyethylene containers are labeled as A) Original nest site; B) Insecticide application site; C to F) Potential budding sites. 

There were some mixed effects observed between treatments and treatment rates. But overall, the researchers found that subcolony size did not significant affect the Pharoah ants’ response to chemical disturbance. Meaning that the Pharaoh ant’s response to a chemical disturbance didn’t change based on how large or small a subcolony was. They also found that Zevo had a higher probability of causing budding when applied at the highest rate, whereas Raid and Black Flag increased the probability of budding at lower application rates.  

When looking at mortality numbers, researchers found that more dead ants were recorded in the Raid and Black Flag treatment groups than in the Zevo treatment group. They interpreted this to suggest that Zevo, an essential-oil based product, exhibited a stronger repellency effect while Raid and Black Flag (pyrethroid-based products) demonstrated stronger contact toxicity. Which is in line with other findings that pyrethroids act as contact irritants rather than spatial repellents like essential oils do.  

So, to summarize, this research showed that these aerosol products do influence Pharoah ant movement and could cause budding. Collectively, these findings suggest that aerosol applications could make management efforts more challenging for pest control professionals trying to deal with an already problematic pest. And, that budding may be delayed depending on how recently the aerosol was applied. Their study also reinforces the importance inspection and monitoring when dealing with Pharoah ants to avoid the unintended consequence of applying a repellent product on or near trailing Pharoah ants.