Asian citrus psyllid (Diaphorina citri) is found in three morphological types with different colored abdomens: blue, gray, and yellow. When harboring the bacteria that causes citrus greening disease, the insect increases its level of the protein hemocyanin, which results in a blue-colored belly. (Photo credit: Michelle Cilia, USDA ARS)

What do horseshoe crabs have to do with citrus greening disease? Well, nothing really. But the protein that famously gives horseshoe crabs their blue blood has been found at increased levels in some Asian citrus psyllids (Diaphorina citri), which researchers suspect is evidence of the insect’s immune response to the citrus greening disease bacteria that it carries and spreads to citrus trees.

D. citri is found in three morphological types with different colored bellies: gray, yellow, and blue. A research team led by Michelle Cilia, a research molecular biologist at the U.S. Department of Agriculture Agricultural Research Service (USDA ARS) and assistant professor at the Boyce Thompson Institute in Ithaca, New York, found that the psyllids increase their production of the oxygen-transporting protein hemocyanin when harboring the bacteria Candidatus Liberibacter asiaticus, which causes citrus greening disease. It’s the same protein found in horseshoe crabs and other mollusks and crustaceans, and it gives D. citri a blue-colored abdomen. Their study is published in Royal Society Open Science.

“The study is allowing you to look at your population of insects and say something about the immune system of the insect based on its color,” John Ramsey, a USDA ARS postdoctoral associate and lead author on the study, said in a press release. “There’s the possibility that this could be a useful part of grove surveillance.”

The study found that hemocyanin interacts with a protein involved in a metabolic pathway in the bacteria, which scientists have targeted in the past when developing antibiotics. The researchers now plan to test whether the different color morphs of D. citri spread citrus greening disease at different levels and whether the hemocyanin protein can be targeted to prevent the psyllid from spreading the bacteria.

Read more: “Protein interaction networks at the host–microbe interface in Diaphorina citri, the insect vector of the citrus greening pathogen,” Royal Society Open Science