Psyllids are small sap-sucking insects. Their saliva is toxic to tomatoes and potatoes and can cause them serious damage. Unless you know what you’re looking for and are checking regularly, the insects can go undetected until it’s too late. Here are some psyllid-detecting tips.
Examine the undersides of tomato and potato leaves; that’s where the bulk of psyllids hang out. Eggs are small, orange-yellow and “ stalked” (held upright by a little pedestal). See if you can find the yellowish nymphs (young). They look like tiny, plastic flying saucers. Following egg hatch, nymphs crawl around the leaf, looking for a place to settle. When satisfied with their choice, they hunker down, feed and remain stationary. So they’re not going to jump or fly away while you’re looking for them.
Unfortunately as nymphs mature, they turn pale green, making them harder to detect. Fortunately, psyllids produce easily-seen “psyllid sugar” (a polite euphemism for their poop). So if you don’t see the beasts but find sugar on plant leaves, you know they’re around…and feeding!
Unchecked, these insects cause plant symptoms known as “psyllid yellows”. Upper leaves turn yellow with a purple tinge to veins and margins. Leaves become stiff and leathery. Tomato plants either produce an abundance of tasteless fruit or produce no fruit at all. Potato plants develop lots of small, weirdly-shaped tubers that may actually sprout underground. Needless to say, the crop is ruined.
So what’s a gardener to do? Check tomato and potato plants regularly. Inspect eggplants and peppers, too. Psyllids can feed on these plants, yet not cause significant damage. Apply insecticidal soaps, sulfur dust (if you can get it coated on the leaf undersides) or insecticides containing permethrin or esfenvalerate. Always follow label directions when using any of these products.
See this Colorado State University extension fact sheet for more details:
http://www.ext.colostate.edu/pubs/insect/05540.html
Here's an article from the Boulder Daily Camera also.