It's caterpillar poop. Get rid of the catties and you will have no more problems. Having said which, that is no easy task. Most chemicals that kill them quickly also kill your nasturtiums whilst good enviro-friendly stuff like Neem takes an age by which time they will be remnants. I use a pair of scissors. Either I cut the catties in half if they are big ones or the leaf if there are a lot of littluns. Good luck. Ian.

Not that I don't believe you, but I've never seen tent caterpillars on nasturtium! Anyway, you could break open the "nest" while they are inside (during the day), thus exposing them to birds and other enemies. Also see below excerpt from Golden Harvest Organics. Btk (bacillus thuringiensis kuratski) is an organic, biological control, not a synthetic chemical, that can be effective and, as noted, controls all types of caterpillars ... which would include those of swallowtails, monarchs, and a host of other butterflies and moths that you might like to have around.

"Tent Caterpillars: The easiest control for trees up to 13 to 16 feet tall is to use a large stick, broom handle, etc. and when the caterpillars are in their nests (usually at noon or during rains) poke the stick into the center of the nest then roll the stick. The silk fibers of the nest will adhere to the stick and as you roll the stick, the silk will wrap the caterpillars up tight in their nest. Then slide the nest and caterpillars off the stick with your foot and squash the whole mass.

For smaller trees or shrubs wash the caterpillars off the leaves with a garden hose and squash them with your foot when they land on the lawn.

In fall and winter destroy egg mass that can be seen on branches about the size of a pencil.

Btk a bacterial spray which when applied to the leaves being eaten by the caterpillars, poisons the caterpillars, killing them usually in about 12 hours or less. Bt only kills caterpillars of the moth and butterfly family and does not harm insects of other families which are beneficial including many insects that may eat the tent caterpillars. Non toxic to people, critters and birds. BTK works on all caterpillars."

Here is a link that might be useful: Golden Harvest Organics - Insect Control

Should be noted that BTK will not kill sawfly larva (which look like caterpillars)......so it is important to identify which it is before applying BTK......

Here is a link that might be useful: BTK