Entomology professor Raymond St. Leger (pictured) has discovered how to use scorpion genes to create a hyper-virulent fungus that can kill specific insect pests, including the mosquitoes that carry malaria and a beetle that destroys coffee crops, and which does not contaminate the environment as chemical pesticides do. 

In the November 2007 issue of the journal Nature Biotechnology, Professor St. Leger  describes how he was able to bioengineer a new version of a fungus (Metarhizium anisopliae) that can inject specific insects with the venom of a scorpion (Androctonus australis insect neurotoxin, or AaIT), killing them within a few days.

Adapting the Fungus to Kill Like a Scorpion

“Scorpions have toxins that are superbly adapted to killing insects,” explains St. Leger.  “A scorpion kills by stabbing into its prey, so we were looking for a way to get the toxin into the insect that doesn’t involve a scorpion. The funguses are really good at that because they are naturally infective and function like tiny hypodermic needles. They land on the insect’s outer surface (called the cuticle) and insert little tubes (called hyphae) and grow within the insect.”

The engineered fungus is nine times more virulent and one of its effects is to cause the muscles to contract so that the wings are outstretched when the insect dies. The main image shows the fungus in the blood of the insect at left in glowing green because in addition to the scorpion toxin it is also expressing the green fluorescent gene (from a jelly fish).

Increasing the Killing Power and Speed

The naturally occurring M. anisopliae fungus and other strains like it are already being used to control agricultural pests and mosquitoes, but their effectiveness has been limited in comparison to chemical pesticides.  Yet unlike chemical pesticides, these fungi can be used to target specific insects and do not pose a threat to the environment.

 In Australia, the fungus is sprayed from airplanes to target locusts and grasshoppers that decimate food crops. In Africa, the spores of the M. anisopliae fungus are put on sheets and hung inside houses to kill mosquitoes.

"The problem is it takes quite a few fungal spores to kill the mosquito, and it is slow," says St. Leger. "It reduces the number of mosquito bites that people get, but it doesn't keep people from getting malaria or dengue. We're trying to get a supercharged, hypervirulent fungus that will take out the mosquitoes quickly."

Fighting Malaria by Attacking the Plasmodium

“The problem is that the fungus doesn’t have the killing power over the mosquitoes,” says St. Leger. “It reduces the amount of mosquito bites that people get, but it doesn’t keep people from getting malaria or dengue.  We’re trying to get a supercharged, hypervirulent fungus that will take out the mosquitoes quickly.”  He has also discovered a way for the fungus to kill the malaria-causing microorganism (called plasmodium) that is carried in mosquitoes so that the mosquito can no longer transmit disease, even if it bites, before it dies.

Protecting Valuable Crops

Likewise, St. Leger is looking at the possibility of using the enhanced fungus to attack the coffee berry borer, an invasive beetle that causes severe damage to organic coffee crops in Colombia and other parts of Latin America.  After oil, coffee is largest legally traded commodity in the world, so the industry is eager to develop biopesticides that will protect the valuable crop.

Utilizing the Scorpion DNA

To produce this new fungus, St. Leger created a synthetic scorpion gene which he inserted into the M. anisopliae fungus.  “You can’t just take out the scorpion gene and put it into the fungus.  You have to turn that piece of DNA into something that the M. anisopliae can use properly,” he explains. 

He also had to create what he calls an “on/off switch” in front of the gene so that the fungus will produce the scorpion toxin only when it is in the blood of the insect.  "The fungus will never produce it under any other circumstances." he says.

A Cost-Effective Pesticide

St. Leger tested the infectivity of the transgenic fungus against mosquitoes, caterpillars and the coffee borer beetle. It was nine times more virulent than the wild M. anisopliae in killing mosquitoes, 22 times more virulent to caterpillars, and 30 times more virulent to the coffee borer beetle.

St. Leger believes this supercharged, pathogenic fungus has great potential to become a cost effective biopesticide that can kill using far fewer spores than the wild M. anisopliae fungus. He is currently using a range of genes, including scorpion toxins, to create additional biocontrol agents that are also highly specific to important pest species.  

Original press release: Scorpion Toxin Makes Fungus Deadly to Insect Pests
News coverage:
Scorpion toxin can target troublesome pest
Fungus has sting in its tail