Now that I've grossed most of you out with my last week's pick, I think I'll go ahead and make this week's pest a little easier to look at.
The image looks like a mustard plant with nice, yellow flowers. But alas! It is actually the product of this week's parasite, the rust fungus Puccinia monoica.
The fungus infects mustard plants and completely changes their 'behavior' to facilitate its own reproduction. Firstly, the fungus infects the plant as a whole, stealing nutrients and generally acting like a parasite. But Puccinia is a sexual fungus - to reproduce it must combine genes with another member of its species. Of course, it's hard to mate when you're stuck on a stationary plant. So the fungus has evolved a unique (and impressive) means of getting laid.
The first thing it does is emit hormones which cause the host to stop flowering. Then it makes the infected plant create pseudoflowers from clusters of leaves. The pseudoflowers look almost identical to natural flowers, even in untraviolet light. And as if looks alone weren't enough to get the job done, the fungus' fake flowers also emit a strong scent and are sticky with a sweet substance that attracts insects. That, in the end, is the goal of these amazing mimics. Bees and other pollinating insects feed on the pseudoflowers, and in the process, they pick up the sperm and eggs produced by the fungus much like pollen. These creatures then land on another fake flower, thus transferring the genetic material and allowing the fungus to reproduce as well as infect other plants.
It's actually quite remarkable how well the fungus has managed to imatate flowers and get itself spread. Imagine a parasite spreading through the human population by castrating us and convincingly reproducing the look of our sexual organs... Maybe like a Cymothea, but not for our tongues. *shudder* If that's not sci-fi freaky, I don't know what is.