Thursday, January 17, 2019

A tiny wasp with enormous numbers of descendants - Polyembrionic Copidosom sp.

I am still trying to keep my basil plant alive, through short days and some freezing nights. But most of the larger leaves are falling off anyway. So now several dry, rolled up ones became very obvious. Each contained a cocoon - similar to that of the green noctuid moth caterpillars that sometimes compete with me for the basil leaves.
But the cocoons were quite thin, likely unfinished, and inside was a rather repulsive looking, thick sausage with no recognizable organisation. 

Cocoons of noctuid moth caterpillars on basil. The caterpillars were killed by parasites
The casing was stretched thin enough to be transparent, so I could see that it was stuffed full of little grains, like rice, but each only less than 0.5mm long.

Under magnification, little eyes became visible on those white grains. So probably really parasites. Larval (or pupal) wasps rather than flies, because those eyed creatures did not look like headless maggots. 

Copidosoma spp, Pupae
But so many? I had never seen anything like it. So I posted them on and my Facebook Group SW US Arthropods. 
Charles Melton and Dennis Haines had the answer quickly: 

Copidosoma spp, adult female
A Chalcid wasp in the genus Copidosoma (Pentalitomastixwas behind this amazing find.
The tiny, 1.5mm long wasp female produced just a few eggs, and probably even spread those among the three affected caterpillars that I found.  But she did not actually attack the grown caterpillar, she had laid her eggs early, right when a moth female deposited her eggs on my basil plant.
The wasp eggs multiplied then by cloning into as many as a few thousand (polyembryony). The larvae didn't begin growing until the caterpillar has reached a certain size. So they all uniformly ended up killing their caterpillar just as it was about to spin a cocoon and pupate. 
I collected two of the cocoons to see what will emerge. Dennis Haines remarked that these wasps are obviously interesting for researchers working on biological control, but they are difficult to raise in captivity. 
The sex of the larvae seems to be determined by temperature ... just one more fascinating aspect.

12 days after I discovered the caterpillar full of pupae, tiny wasps are hatching. This one is sitting next to some writing in font size 12, so you see that they are barely 1mm long.

A day later, the container was alive with thousands of little wasps, and some were already mating. So I quickly released them on my lattice plants that often get eaten by noctuid caterpillars. I wish the little wasp females luck in finding any moth eggs in the middle of winter!

Better here:

PS: there is already another generation pupating in another noctuid cat, again on basil. This is now happening in February, while we actually had a very rare snow fall righr here. Picture Rocks, Pima Co, AZ elev ca 2500 feet. 

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