Is it possible to "weld" holes in my merino shirts?

“Mystery holes” in wool clothing is usually caused by those dreaded moths! Keep all wool clothing in sealed plastic bags until use. I learned this the hard way and discovered that moths out west seem more prevalent than back east. Lost several wool scarves and two sweaters to them. :o(

Judicious sewing is the fast way to close these holes but a good re-weaver can do it also. Had it done once to a prized Pendleton shirt and it was perfect. They take some threads from the shirt tail and weave then to close the hole. Not cheap but works well and does not come out with washing.

SeamGrip should work.  It may or may not be satisfactory to you.

Sometimes, you can put plastic tape over it on one side, SeamGrip on the other, let it dry for 24 hours, remove tape, maybe put some more SeamGrip on if it doesn’t completely cover

Ah, the lost art of “darning”… Used to be, out on the prairie in the 1800s, that you couldn’t just pop over to REI or Macy’s or whatever when your sock sprouted a hole.  You had to darn it, which involved essentially weaving wool or thread into/out of the sides of the hole to “recreate” fabric where the hole was.

THIS article shows the basic idea — there are also some YouTube videos that you can Google.

If you see moths flying in your house, they probably aren’t clothes moths, but pantry pests — the type that infests flour and grains. Clothes moths don’t like light and are so secretive that you’ll probably never see them. What’s more, the adult moths won’t do any harm. Damage to woolens is actually done by the larvae of two types of insects: clothes moths and carpet beetles.Both insects lay eggs in secluded spots with plenty of food — wool, fur, down, shed pet dander, and other animal-based materials.Moth and beetle larvae shun bright light, so they rarely attack frequently worn clothing or heavily trafficked carpets.Before you pack up winter clothing for storage, wash or dry-clean garments that have been worn. This rids them of moth and beetle eggs and also eliminates perspiration remnants and food spills, which attract and nourish pests. Moths and beetles don’t eat items made of synthetic or cotton fabrics, but you should clean those, too, if you store them with woolens.You won’t likely see clothes moths, but if you find holes, you know you have a problem.Remove and treat all infested material. You might throw away the most damaged clothing. Dry-clean or launder items you keep; freezing also eradicates pests: Put items in sealed plastic bags, squeeze out air, and freeze for a few days. Take the bags out, let them return to room temperature, and then repeat. In case of condensation, let clothes air out before storing again.

I thought I might have moths so I got https://www.amazon.com/Safer-Brand-05140-Pantry-Traps/dp/B000HJ7AIY/ref=sr_1_1?s=lawn-garden&ie=UTF8&qid=1526503679&sr=1-1&keywords=Safer+Brand+05140+The+Pantry+Pest+Trap%2C+2+Moth+Traps&dpID=51PlW3pbB5L&preST=_SY300_QL70_&dpSrc=srch

also one for clothes moths

after a couple weeks the traps had no moths so I don’t think I have any

Another possibility, if washing merino clothes with other clothes, is that the zippers or hooks on other clothing are creating holes. This happened to me twice where I noticed no holes before washing and a small hole or two afterwards. I now wash merino in delicates bags (mesh with a zipper) if I’m washing them with other clothes, and started the practice of closing all zippers before starting a load of laundry. Hand washing them is another good option.

I’ve found with the very small holes (like a pencil point) that a few quick stitches and a knot creates less bulk than darning them (since I’m using polyester thread and not undoing merino threads from the fabric as Eric explains a professional would do). These stitches have lasted for me.  I do darn socks though.

In my experience, most of the holes are caused by the mechanical action of wearing the clothes and or the mechanical stress of the washing machine.  Patagonia for example, when it sold the very lightweight merino recommended hand washing the items and drying them by rolling them inside a towel. Smartwool, Patagonia, and even Ibex before it folded were bringing to market weaves that were 8 to 10% nylon to address this problem.

You do not say which weight of merino wool you are having problems with. I find that with the lightest weight, for example Smartwool 150 micron, my local laundry has very thin iron on patches that last for months. Another fix is Gear Aid (formerly branded McNetts) Tenancious Tape.  Darning the light 150 micron just seems to pull the weave and cause another hole elsewhere. When using the tape or a patch from the laundry, you need to cover about 1 inch diameter around the hole.
Cheers!

I don’t have moths but I do have silverfish

And i think  they ate holes in wool dress pants, blazer, hiking shirt