Monsoon brings some unwanted pests to life and forces others inside
University of Arizona entomologist Lucy Li, who coordinates the university Public Health Integrated Pest Management program, highlights a few of the usual suspects.
Potted plants are a prime target for mosquito moms looking for places to lay eggs. In 2023, University of Arizona entomologist Kathleen Walker, an assistant Extension specialist, studied the effectiveness of hormone to control the spread of urban mosquitoes.
Brad Poole, Cooperative Extension
Through much of Arizona in recent weeks, monsoon storms began sweeping across the landscape, bringing a collective sigh of relief and broad smiles.
But the rain can also bring unwanted creepy crawlies to life or out of the woodwork, so to speak. Our late summer weather offers a breeding frenzy for some pests and forces others inside your house, where you don’t otherwise find them.
There are ways you can counter the onslaught, said Lucy Li, an associate in Extension at University of Arizona.
Li, an entomologist in the Department of Entomology, coordinates the university Public Health Integrated Pest Management program. The goal of the multidisciplinary team is to improve public health by reducing environmental and health risks.
If you have unwanted guests, our Home Pest page can help you figure out who they are and what to do about it. Here are some common monsoon visitors who overstay their welcome:
Cockroaches
Summer rain and construction can force the American cockroach out of sewers and into homes.
This summer some parts of Arizona have seen an uptick in appearances by these urban dwellers, Li said.
“They are generally sewer cockroaches. I think because of construction and also the rain and lots of other conditions, those cockroaches somehow are just kind of invading people's homes,” she said.
Keeping drains covered can reduce risk. Male cockroaches are attracted to light, so keeping doors and windows sealed can deter them from coming into your home at night. Clean up food scraps and keep compost piles away from the house.
Traps and bait stations can help keep them at bay, if they get inside.
For more information about cockroaches, see this Extension publication.
Mosquitoes
Mosquitoes peak in Arizona between June and October.
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In Arizona, mosquitoes, which lay eggs in standing water, peak during monsoon between June and October, Li said.
Rain fills retention basins and other low spots in the landscape, as well as buckets, old tires, potted plants and other accidental backyard breeding receptacles. One neglected swimming pool can leave an entire neighborhood itching and swatting and could become a health hazard.
Mosquitoes can grow from eggs to adults in about 10 days, so weekly checks around your house for standing water can help, Li said.
Some communities monitor mosquito populations with traps to keep tabs on the potential public health threat, she said.
“They count the numbers in each trap, and identify what kind of mosquitoes they are dealing with, because some mosquitoes can transmit disease-causing pathogens to humans, for example West Nile virus,” Li said.
See this Extension publication or this issue of Extensions IPM newsletter to learn more about mosquitoes.
Ants and termites
Summer is breeding season for termites, so you might see alates, or winged adults, swarming to create a new colony.
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Ants and termites with wings, known as alates, often leave their nests during monsoon. Pest technicians have reported seeing swarming termites, and Li had a brief incursion of tiny black ant alates in her own home, she said.
“Right now, it's the reproductive period for termites and ants. They are just swarming so that they can start a new colony, so people will be seeing more of those,” she said.
Several species of termites are native to Arizona. Learn about the three basic types in this Extension publication, and learn how to combat them in this one or this one.
Colorado River toads
Though technically not a considered a pest, Colorado River toads emerge after monsoon rains and can be deadly to people and pets.
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This native desert amphibian, Incilius alvarius, is technically not a pest, but it can be deadly for people and pets. The 3-7 inch native southern Arizona species secretes a defensive poison from glands on the sides of its head.
“Dog owners have to be especially careful. One of my colleagues' dogs played with a Colorado River toad, and unfortunately got killed,” Li said.
Secretions from the toad have psychoactive properties, and the substance is a Schedule I drug under the Controlled Substances Act. The toads are endemic to southern Arizona and often emerge after rain.
If you see the toads, do not handle them or allow pets to.
Mesquite bugs – false alarm
Cooperative Extension gets calls regularly about mesquite bug nymphs, which look scary but are harmless to people and plants.
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One insect that emerges during monsoon that Li always gets inquiries about is not a pest at all – Thasus neocalifornicus, the giant mesquite bug. The nymph of this insect, which eats the sap from mesquite trees, is bright red with striking white markings. They collect in large numbers on mesquite trees, but they’re harmless to the trees and people, Li said.
“It's a very beautiful sight to see, but people get scared. We get inquiries, people say, ‘Oh, those bugs look scary,’ but they are not harmful to humans. They don't kill the plants. So literally, just enjoy it when you can,” Li said.
Exactly who is the pest here?
Although we consider many insects and other wildlife pests, we came into their environment, not the other way around. Mosquitoes, for example, are a natural part of the environment in Arizona. The larvae provide food for fish and other aquatic life, and adults offer a feast for bats and birds, Li said.
“It's not fair. So that's why, as entomologists, we try to teach people to be appreciative, to be grateful. You know, even though we talk about insects, 99 percent of them are just existing, just like humans,” Li said.
For more information about pest management, including in public spaces, see the Arizona Pest Management Center website.
To stay up to date on pest management in Arizona, subscribe to Cooperative Extension’s Integrated Pest Management newsletter.