Most of the time, our first impulse when we encounter insects on a typical day is to either kill the pests or do our best to avoid them. This year, with the spread of the mosquito-borne Zika virus from South America to Florida and points northward, the insects’ threats to our health and safety are even more prominent in the public consciousness.
Insects have contributed to some of the worst public health events in human history, and it’s important to understand the dangers they pose and to know how to reduce the chances of contracting an insect-borne disease. But, insects make many valuable contributions to environmental welfare as well.
On this episode of Kentucky Health, host Dr. Wayne Tuckson speaks with an international expert on entomology from the University of Kentucky and a Louisville entrepreneur and butterfly farm owner about the historical impact of insects on public health, the current Zika outbreak, and the benefits insects bring to gardening and farming.
Dr. Grayson Brown, Ph.D., is the director of the Public Health Entomology Laboratory at the University of Kentucky and the former president of the Entomological Society of America. Blair Leano-Helvey owns Idlewild Butterfly Farm in Louisville and Entomology Solutions, Inc., which educates on the use of biological organisms as a means of pest control.
Insect-borne diseases caused some of the most severe plagues in recorded history. Leano-Helvey recounts the Black Death, the bubonic plague which wiped out roughly half of the population in Europe during the 14th century. That scourge was transmitted to the European continent by fleas carried by rodents from Asia, and its spread was intensified by the lack of personal hygiene of nearly the entire population of that time.
Brown says that the bubonic plague still exists, and is present in the southwest United States. This plague is carried by fleas on prairie dogs, and Brown is concerned that transient people crossing the border from Mexico are susceptible to contracting the disease.
Yellow fever, arrived in this country via the slave trade and was transmitted by mosquitoes from Africa. The diesease spread through most of the eastern half of the North American continent in the 18th and 19th centuries, with serious outbreaks cities as far north as New York City and Philadelphia.
“Here in Kentucky, we had several large outbreaks,” Brown says, “with the biggest one being in 1878 along the Mississippi River in the Jackson Purchase area, as far north as Paducah. Almost wiped out those towns in that area.”
Brown credits research by the Cuban physician Carlos Finlay, and subsequent larger studies by renowned U.S. physician Walter Reed, for isolating the mosquito as the vector (transmitter) of the yellow fever virus. Due to a massive mosquito eradication campaign, incidences of yellow fever in the Western Hemisphere’s subtropical regions were reduced in the early years of the 20th century, allowing the U.S. to complete the Panama Canal. A vaccine was developed in the 1940s.
Current Threats: From Zika to Chagas
Mosquitoes are also the vectors of the disease on everyone’s minds this year. Brown details the history of the Zika virus, which is carried by the mosquito Aedes aegypti and can cause serious birth defects such as microcephaly (small head size) in children whose mothers are infected. The virus was discovered 1947 among rhesus monkeys in Uganda, and the first human cases were discovered in the late 1960s.
Confined to Africa for decades, the Zika virus spread east in the mid-2000s and mutated. “It came to South America last year,” Brown says, “possibly a little bit earlier, and really radiated in terms of the strains and the symptomatology that it causes, including the now infamous microcephaly.”
The Aedes aegypti mosquito used to be the principal mosquito that bit people in Kentucky, Brown says, but that changed in the mid-1980s when it was pushed out by an invasive species, the Asian tiger mosquito. That mosquito does not carry Zika in the U.S., according to Brown.
Only female Aedes aegypti transmit the Zika virus, Brown says, doing so when they bite humans to acquire protein for their eggs. Currently in South America, a massive male mosquito sterilization project is underway to try and halt the population growth of the Aedes aegypti. Males are sterilized by radiation, Brown says, or by genetically modifying male mosquitoes in their larval stage. These efforts, even if successful, will likely be met with adaptation by the species in the future, Brown predicts.
In the U.S., mosquito control has largely been applied by spraying of insecticides, but Brown says that ultimately, success will depend on public education and involvement. Removing any outdoor equipment that can hold standing water is crucial to stop mosquito breeding, he instructs.
“Because Aedes aegypti is a container breeder – and by container, I mean human-created containers – the individual homeowners generally are breeding the mosquitoes that are biting them,” Brown says. “So, because the mosquitoes breed in these really small containers, it’s not practical to expect local mosquito control authorities to treat every one of those little things. We really need to have a partnership, a collaboration between the local mosquito control authorities and individual citizens. People need to take responsibility for mosquitoes that are breeding in their own yards.”
A lesser-known but growing threat, according to Brown, is the infectious disease chagas. This disease, which originated in South America and has spread north, is carried by a cone-shaped insect known as the Triatomine bug, or kissing bug.
“It’s called the kissing bug because it has the habit of crawling across the ceiling at night when a person is asleep, and dropping down onto their chest, and then following the CO2 up to their lip and then feeding right there,” Brown says.
The kissing bug often defecates right after feeding, and if a person happens to scratch or rub their wound after waking and mixes in the insect’s waste into their bloodstream, they risk becoming infected with a lifelong, debilitating disease carried by the parasite Trypanosoma cruzi.
The Benefits of Insects in Pest Control
Leano-Helvey helps her clients, including nurseries, greenhouses, and farmers, devise methods of pest population control that shirk pesticides and instead enlist a variety of what she calls “beneficials” – insects that either feed on or parasitize the pest organism.
The good thing about biological pest control is that the pest cannot become resistant to the predator, or to the parasitoid,” she says. “… in the world of biologicals it’s a numbers game.”
Typically, it takes a year for a client to fully transition to biological pest control, Leano-Helvey says. First, their environmental area must be rid of any pre-existing chemical residue. Then, a plan is devised to release the beneficial insects at times of maximum impact. For greenhouses, a schedule follows the seasonal cycle, ramping up the release of beneficial insects when spring commences and pest organisms start to reproduce in greater numbers.
Many of the most useful beneficial insects in biological pest control are not well-known to the general public, Leano-Helvey says, but they are very effective for reducing specific pest populations. “Ladybugs and praying mantis are the poster children for bio control… and those are two of the beneficial insects that are the last ones I recommend. People like them, because they can see them, but a lot of your biological insects are very tiny, almost microscopic.”
Brown notes that biological control methods are not effective against mosquitoes. In general, he is optimistic about efforts to fight against the spread of Zika and other insect-borne diseases, although he cautions that methods available to developed countries such as the U.S. are lacking in many parts of the world where these maladies are much more devastating.
“The key thing when we’re talking about mosquito control, and about different control methods, GMOs, sterile males, insecticides, whatever, is that there is no one silver bullet. You cannot find one control method,” he says. “No matter how well it works, you cannot use that one thing over and over and over, and expect it to persist. Sooner or later, a resistance mechanism will arise and then you’ll lose that agent, and it will be gone.”
