What’s This Weird Bug?

20180812A Denise Lalonde

Wasps infested with white muscadine. Source: Denise Lalonde

Question: I’m sending you a photo of some weird bugs I found on my astilbes. Can you identify them? Are they harmful?

Denise Lalonde

Answer: They are certainly weird, but they aren’t harmful … at least not to your plants.

What you’ve photographed are wasps killed by a fungal disease called white muscadine (Beauveria bassiana). This cosmopolitan disease is deadly to insects, but almost never touches warm-blooded animals, so there is no risk to humans or pets.

20180812B atrium.lib.uoguelph.ca.jpg

Spore cases of white muscadine, highly magnified. Source: atrium.lib.uoguelph.ca

White muscadine spores overwinter in the soil. If an insect comes in contact with one of them, it sprouts on its body, then rapidly penetrates it, attacking the internal organs. The fungus eventually kills the insect, using it as a source of food, and a white-to-yellowish mold consisting of spore cases forms on the outside of the insect’s body. That’s what you saw.

Before it dies, however, the insect thus attacked spreads the spores through its movements, therefore helping to ensure the survival of the fungus. In the case of social insects like wasps, bees, ants and termites, the infected insect often transmits them to its fellow workers and this can annihilate the entire colony.

Sometimes the dying insects display a bizarre behavior. Instead of dropping to the ground to die out of sight, they settle out in the open, up on a plant. This means the disease spores will be more readily picked up and carried by the wind, increasing the spread of the disease. The theory is that the fungus actually takes control of the insect’s brain in its final hours, telling it where to go to die.

It sounds like something out of a horror film: the insect version of Invasion of the Body Snatchers!

Not all strains of white muscadine seem to have this effect, but there are hundreds (indeed, probably thousands!) of strains of this very common fungus and who knows what possibilities they hold!

Potentially Useful

White muscadine is certainly harmful to many insects and other arthropods, but is also proving itself useful to humans.

The potential of white muscadine in integrated pest management (IPM) has been known for a long time. Already in 1906, the possibility of using this disease to control harmful pests was brought up at the Congress of the French Association for the Advancement of Science. However, although progress has been made since, white muscadine is still not commonly used as a pesticide.

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Bed bug killed by white muscadine. Source: KDS444, Wikimedia Commons.

It does have some applications, though. For example, it’s used in the biological control of termites, banana weevils and red palm weevils and is being studied for the control of grasshoppers, bed bugs and malaria-carrying mosquitoes.

As mentioned, there are many strains of this disease. Some are generalists and attack a wide range of arthropods. They would probably not make good choices in IPM, because they attack both beneficial and harmful insects. Others, on the other hand, are quite host specific. Strain Bba5653, for example, attacks only the caterpillars of the diamondback moth (Plutella xylostella), harmful to cabbages and other crucifers, plus a few closely related moths. These are the strains scientists are looking the most seriously into.

White muscadine: a very odd fungus that could one day be among the most useful pesticides in the organic gardener’s arsenal. The next time you see a dead insect, take a closer look!20180812A Denise Lalonde


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