These appear to be the “shed skins” from sap sucking insects called Aphids. These insects molt thru several developmental stages, leaving their old exoskeletons on the leaves where they are or have been feeding. They typically feed on the newest growth and sometimes flower buds, but you’ll also find them on the underside of the leaves, especially if it gets really warm/hot. If you’ve not been spraying the undersides of the leaves, that’s why they keep coming back. They also excrete the sugary plant sap (we call this honeydew) which makes the plant parts sticky. This excrement will also attract ants (they eat the excrement) and it provides an ideal environment for black sooty mold to grow. This mold won’t infect the plant but it does cover the leaves and can reduce photosynthesis so its a good idea to wash off (with a high pressure spray of water) the honeydew and any sooty mold that shows up. We’ve included a link for your reference about how to control these common plant pests. We recommend either horticultural oils (only use in the cool morning hours when the plants are also well watered to avoid burning the plants) or insecticidal soaps, but if these don’t do the trick with proper and thorough application of the upper and undersides of the leaves, you can ask your local garden center for something systemic.
http://www.ipm.ucdavis.edu/QT/aphidscard.html
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