Can't help you on the exact culprit, but you could try a simple spray. I soak garlic and hot pepper slices 24 hours, strain out the water and mix in several drops of Dr Bronners peppermint soap
This can be sprayed on seedlings and soil evening or early morning, avoid using at peak temperatures. This could be improve the survival rate.
my daughter the entomologist has just left for work so i can't tell you exact name (also without seeing the adults it is hard to tell anyway) but it's some type of pseudo-scale insect (mealybug, here in Brazil we call it cochinilha but actually it's a pseudococcidae).
in my yard they especially like my citrus but I can find them on almost anything.
I use a weak soapy spray (small bit of dish soap in water, repeat daily).
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