Question
Hi, I am Damian from Malta, up till a couple of weeks ago I had several Geranium pot plants, all successfully cultivated from various cuttings from friends and relatives. These (except for one) are of the scented leaf variety and ranged from white through several shades of pink to a deep red. The other specimen has leaves which are not scented and have sharp pointed ends. A few weeks ago I started noticing that some of the branches from the stems (of the scented leaf type) had what appeared to be holes in them and some of the leaves started displaying areas which had holes or were thinner than the surrounding area and transparent. The leaves on the afflicted stems started yellowing and falling off whilst the stems themselves seemed to be hardening, getting brown and shrivelling dry. I lost several plants this way since the whole plant started getting weaker. Eventually I took a scalpel to the plants in an attempt to isolate the afflicted portions. I found that the stems were dead and almost totally hollow, blackened inside and containing a black granular substance (excreta of some organism?)instead of the normal plant tissue. What I thought were holes in the stems were indeed so, possibly the point of entry of what was causing this. Unfortunately, my endeavours with the scalpel revealed so much damage that most of my plants were reduced to nothing and were not salvageable since they were relatively small, being less than two years old. During this exercise I noticed a number of caterpillar like organisms on the plants which I removed. They ranged from green in colour to light brown or off-white with a red stripe running down the back and were between 4 - 9mm in length. Some of them were dead and black, possibly the result of an earlier attempt a few days ago to kill them with a solution of water and domestic dishwashing soap (works wonders on roses). I am not sure whether this is the work of the plume moth or the tobacco bug and I don't know whether these species exist in Malta but the only moths I ever noticed around the plants was a normal looking moth with triangular wings (not like the plume moth) which were a maximum of 12mm across each wing. Can identify the culprit and give me some pointers as to how I might save the sorry remains of my plants? How do I prevent this from happening again? By the way, I mentioned the other Geranium which does not have scented leaves bacause it appears to be completely unafflicted.

I am informed that the problem might be a serious one since there appears to be a local infestation in the particular area (Balzan) where I live.

Answer
I preface by saying, I am not familiar with the moths or caterpillars in Malta. I would recommend that you try to contact someone in your area who is. Perhaps an agricultural authority who has heard of similar problems, for exact identification.
You have already found the problem is the caterpillars you described. But it sounds like you have more then one variety. They will not stop destroying your plants until they are killed or fly away.
If you have enough of your plants that are worth saving, then this is what I recommend. First, isolate the infected plants to keep the problem from spreading. Continue to remove the damage, as you have.
Pick off any of the bugs that you see by hand. The rest may be killed by using an insecticidal BT (Bacillus Thuringiensis). Apply on a overcast day or in shade, it breaks down in sunlight. Repeated application will be necessary. Apply on the entire plant, especially under the leaves. When the caterpillar eats it, it freezes the stomach and it starves to death.

Horticultural Oils are also effective. They clog up their system. Again good spray coverage is essential as well as repeating often.
Use Ultrafine oils, also known as Summer oils.

Carefully follow the directions on both of these. They are relatively safe solutions for humans and plants.

As for saving your plants. . . I would wait to see how much remains after you treat the problem. If the plant still looks like it will thrive, then repot it in fresh, good, potting soil. Be sure all of the damaged parts are removed. It sounds like the Cat. is injecting a substance into the plant. They do that.  When this happens, especially to a small plant like yours, it will usually kill the plant. But, if by chance there are still healthy leaves and stems, you may be able to revive it in time.

To prevent it from happening again, would require a way to block the moth from your plants. This can be done with a fine net over the entire plants, or repetitive spraying of insecticide. Be very aware of your plants and spot problems before they get this far, and act quickly. Examine them every day. Keep your plants VERY healthy. Bugs and diseases usually attack weak plants.

I was interested in your information about the damage on the scented leaves and not the unscented. Perhaps, if this problem is continuous, it would be wise to grow only unscented varieties.