Bye Bye, OJ: How a Blue-Bellied Insect Helps Bacteria Kill Off Citrus Crops

A tiny louse is responsible for decimating the citrus industry. Diaphorina citri, the louse in question, better known as the Asian citrus psyllid, harbors and spreads the "Candidatus Liberibacter asiaticus" bacteria that causes citrus greening disease.

It's one of the more serious diseases, if not the most serious disease of citrus fruits. The lice, also known as jumping plant lice, move between trees and infect them with the bacteria they carry, which then kills the citrus trees' productivity.

The disease has affected citrus groves in Asia, Africa, and the US. It's been in California since 2008 and in Florida since 1998, where it has devastated crops so much that some citrus growers have given up planting. The amount of land farmed by Florida citrus growers decreased by an area about the size of Dallas in the eight years between 2004 and 2012. Production dropped by almost 50 percent.

The leaves of diseased trees start to turn yellow and productivity slows down, fruit production decreases, and any fruit produced may be misshapen, remain green at the end (hence, the name "greening"), drop prematurely, and/or taste bitter.

Bye Bye, OJ: How a Blue-Bellied Insect Helps Bacteria Kill Off Citrus Crops — Citrus greening disease in action. *Image by USDA/Flickr*

A research team led by Michelle Cilia, molecular biologist at the USDA's Agricultural Research Service, and an adjunct assistant scientist at the Boyce Thompson Institute for Plant Research, discovered a chemical they think may be a telltale sign that a louse is infected with the citrus greening disease bacteria. Their findings have been published in the journal Royal Society Open Science.

The belly of the lice can appear as blue, gray, or yellow at different times. The changing color tipped off the researchers to a metabolic process that might be occurring within the lice. They found that the blue color is due to hemocyanin, the same chemical that makes blood blue in horseshoe crabs, which has never been seen in an insect before. They also found that hemocyanin is increased when the lice are infected with the bacteria and may somehow be related to the insect's attempt to fight off the bacterial infection.

The Race to Eliminate the Pest

Citrus greening disease is thought to have originated in China in the early 1990s and is also called huánglóngbìng, or yellow dragon disease. Despite efforts to eradicate the Asian citrus psyllid, the infection shows no signs of slowing down.

Current efforts to slow the infection of citrus trees are focused on the lice; Farmers are using pesticides and insect predators to to kill the lice, and the USDA is enforcing mandatory quarantining of infected trees. These predator insects, including parasitic wasps, can kill about 80% of the lice, but the female louse can lay up to 800 eggs and some trees are infested with 40,000 bugs, so those efforts will never be completely successful.

Only a massive and thorough effort will be able to curb this devastating disease. Researchers think that killing off the lice might not even be the best solution. Instead, they're hoping to target the bacteria itself.

A Complex System of Life

The lice are infected with the bacteria when they are in the young immature nymph stage, and the new research suggested that may be due to the fact that adult lice mount a better immune response to the bacteria via the ramping up of hemocyanin production—the nymphs don't fight off the bacteria as well as their adult counterparts.

The hemocyanin fights off the bacteria by messing up the pathway that helps the bacteria produce energy. Not only does this help farmers understand the lifecycle of the disease, but researchers think that antibiotics directed toward this pathway may be a way to block bacterial transmission by the psyllid to the trees.

Cilia and her team believe this approach may provide a longer lasting solution because the insect isn't under pressure to evolve to survive the treatment the way they would respond to pesticides or predatory insects. Since an antibacterial treatment like this targets the bacteria, instead of the lice itself, the insect isn't under attack or killed by the treatment.

On February 6, 2017, the US Department of Agriculture awarded $13.6 million to citrus greening research projects. With another 26% decline projected in Florida's orange crop for the 2016-2017 season, this investment will hopefully help researchers find the right antibiotics to fight the disease.