By Jennifer Kvamme, DVM
The mosquito is the main culprit (vector) responsible for the spread of heartworm disease in dogs and cats. When a mosquito bites a heartworm-infected dog, wolf, coyote, or fox, it can pick up microfilariae (baby heartworms) that circulate in the bloodstream. The heartworm larvae mature within the mosquito for a while, and then when the mosquito bites another animal, it can transmit them to the new host where they finish out their life cycle. Controlling the population of mosquitoes and preventing mosquito bites is part of preventing this deadly disease in dogs and cats. In order to control the population in neighborhoods and backyards, it helps to understand how the mosquito reproduces and develops.
Understanding the Mosquito Life Cycle
There are four stages in the life cycle of a mosquito—egg, larva, pupa, and adult. The first three stages of the life cycle take place in water. Only when they reach the adult stage do mosquitoes become capable of flying. Every species of mosquito requires water to complete its life cycle. The difference is in the type of watery location where certain species lay their eggs—and in how they lay their eggs.
Species and Sex Differences
Some varieties of mosquitoes choose to lay large numbers of single eggs in moist soil that will eventually become flooded after the eggs are deposited. The eggs will hatch after the area has filled with water. Other species lay clumps (rafts) of eggs on the surface of standing bodies of water (tide pools, marshes, pastures, ponds, holes in trees, etc.) that are protected by weeds or structures. The rafts are very small, measuring only about a ¼ inch long but contain hundreds of eggs. The mosquitoes that lay eggs in flood plains are most active in early spring, while those that lay eggs on surface water are more active in the summer.
There are also important differences in the feeding methods of the male and female mosquitoes. Female mosquitoes require a blood meal from humans or animals in order to produce eggs. Males feed only from plants to get energy to survive. The lifespan of female mosquitoes is about a month, depending on the environmental temperature, time of year, and humidity level. The lifespan of male mosquitoes is only about one week. Female mosquitoes can “hibernate” over the winter. Males die when winter arrives, but many eggs containing males (and females) will survive to restart the cycle once warm temperatures return.
The time required for a complete mosquito life cycle depends on the species of mosquito, as well as the environment in which it is developing. In cooler temperatures, it can take about two weeks for some species to complete a life cycle, but in warmer temperatures, this process can take place in ten days. There are some species that manage to complete a life cycle in as little as four days, or even extend it out to as long as a month, depending on the environmental conditions.
When the mosquito eggs are laid, the time it takes for them to hatch will depend on the species of mosquito and where the eggs are deposited. If the mosquito eggs are in moist soil waiting for water to flood the area, the eggs may lay dormant for up to a year before hatching. Mosquito eggs on water surfaces will usually hatch in just a few days, depending on environmental temperatures. The warmer the weather, the faster they will hatch.
Some types of mosquitoes are especially good at laying eggs in areas where people and their pets live. For example, female Aedes aegypti mosquitoes like to lay their eggs on the damp, inner walls of containers like bowls, cups, cans, buckets, flower pots, pools, bird baths, fountains, tires, barrels, vases, etc., that are commonly found in backyards and other neighborhood locations.
When the mosquito eggs hatch, the larvae emerge and make their way to the surface for air. The larvae hang from the surface of the water, using breathing tubes, or siphons, to breathe. Some species of mosquitoes do not have siphons, but lay along the surface of the water to get air. The larvae eat particulate matter, such as algae, in the water, and shed their skins several times (molting) as they grow.
Larvae go through four development stages (called instars), increasing in size after each change. Depending on the species, the time spent as a larva will vary. Water temperature also plays a role in the speed of maturity. Larvae will develop anywhere from five to 14 days, after which they become pupae.
The pupae continue to live in water and are very active. They must stay near the surface in order to breathe air from time to time. Unlike the larvae, the pupae do not eat. It is during the pupal stage that the mosquito develops into an adult. Development takes a few days, and the time necessary for the change to an adult depends on the species and the surrounding environmental temperature. Once development has occurred, the skin of the pupa splits open and the adult mosquito emerges. It remains on the surface of the water for the time it takes for the body to harden and dry.
Adult mosquitoes mate in the first few days after emerging. After drinking a blood meal, the female mosquito finds a shady location to develop her eggs over the next several days. After laying the eggs, the female mosquito may go on the hunt for another blood meal; she can then lay more eggs without the need to mate again. A female mosquito will often lay several batches of eggs before completing her life cycle.
Protecting Pets from Mosquitoes
It only takes the bite of one mosquito to transmit heartworm disease to dogs and cats. Therefore, it is vital that owners protect their pets from this potentially fatal disease. Thankfully, many safe, effective, and easy-to-use heartworm prevention medications are now available. Your veterinarian can recommend the best product based on your pet’s particular needs.
Pet-friendly methods of controlling mosquitoes can help prevent the itchiness and irritation of bites. Drain or remove any containers that can hold water for more than a day or two. Keep pets indoors at dawn and dusk when mosquitoes tend to be most active, and make sure any windows that you keep open are well-screened. Finally, many pet products are available that can help repel mosquitoes, but keep in mind that some, even those that are advertised as being “natural,” can be quite dangerous, especially to cats. Ask your veterinarian for a recommendation if you think your pet could benefit from a mosquito repellant.