A Predator in the Soil
Beneath the surface of soils across the globe, including in Colorado, a microscopic drama unfolds. Entomopathogenic fungi, such as Beauveria bassiana and Metarhizium anisopliae, exist as natural predators of insects. Unlike pathogens that must be ingested, these fungi are contact killers. The infection begins when a fungal spore, or conidium, lands on an insect's outer shell, the cuticle. The spore adheres to the waxy surface using a combination of electrostatic forces and specialized proteins called hydrophobins.
Once attached, the spore germinates, growing a germ tube that presses against the cuticle. The fungus then releases a mix of enzymes, including proteases, chitinases, and lipases, which are specialized to digest the proteins and chitin that form the insect's tough exoskeleton. After breaching this outer defense, the fungus grows into the insect's body cavity, or hemocoel. Inside, it transforms into yeast-like cells called blastospores that multiply rapidly in the nutrient-rich insect blood, or hemolymph. The fungus consumes the insect from the inside out, a process that can take from three to fourteen days, eventually causing death by starvation and tissue destruction. After the host dies, the fungus grows out through the cuticle, covering the mummified corpse in a white or green fuzz and releasing new spores into the environment to infect other insects.
Engineering a Better Biopesticide
The natural insect-killing ability of fungi like Beauveria bassiana has made them valuable tools for biological pest control. They are commercially available in products that target a wide array of agricultural pests, including aphids, thrips, whiteflies, and beetles. These mycoinsecticides are an alternative to chemical pesticides, with a high degree of specificity that leaves many beneficial insects unharmed. Research into these fungi is active at institutions like Colorado State University, focusing on overcoming environmental factors that can limit their effectiveness in the field.
To improve their speed and potency, scientists have turned to genetic engineering. The natural infection process can be slow, which is a drawback for agricultural applications. To shorten the killing time, researchers have modified these fungi to produce potent, insect-specific neurotoxins. One prominent example involves inserting a gene from the North African scorpion Androctonus australis into the fungus Metarhizium. This gene codes for a toxin (AaIT) that blocks sodium channels in insect neurons. Other experiments have used toxins from spiders or integrated genes for enzymes that further degrade the insect cuticle. A Metarhizium strain engineered with a scorpion toxin gene (BjαIT) had a median lethal dose 18.2-fold lower than the wild type and reduced the median lethal time by over 28%. These engineered fungi kill their insect targets much faster, often in a matter of days, providing a more effective and targeted approach to pest management.