I think you're doing what needs to be done. Be aware that sowbugs present no threat to established plants. But seedlings are another deal. Good heavens, don't replace the soil! The leaf mulch was an excellent additive. Plant seeds deeply, and do not water until they have their first leaves. Moist mulch is what attracts them.

And if you want to take it a step further, one of the many slug pellet pesticides (available in both organic and synthetic forms) can help.

Dave

'Do not water until they have their first leaves...'

I don't quite get that instruction. Seeds won't germinate in dry soil. Do you mean don't water after the initial watering when sowing? And 'plant seeds deeply' is also a rather sweeping generalisation. Too deep and they won't germinate at all.

Although woodlice like some moisture I find they are worst where there is relatively dry undecomposed organic matter. Next year when the leaves are more integrated the problem should improve. I have to confess to not mulching seedbeds at all in my climate. It just provides a home for pests.

The idea is that you plant seeds where the moisture is. Obviously you don't plant "too deep". You avoid applying moisture to the surface until the plants are getting past seedling stage. Sowbugs or pillbugs thrive in moisture. I never find them where it is dry. This is a standard recommendation. Just repeating what you can read anywhere.

True, in some climates, that might be hard. A related strategy might be to cover the dry surface seedbed with plastic or paper until the seeds start to come up. That would keep moisture at depth.

Thanks everyone for your responses! I'll keep the existing soil, keep on using the diatomaceous earth, and start Sluggo Plus or other organic pesticide. Follow-on question: if we use leaf mulch again to winterize the beds this November, is there something we can do to minimize their presence in the spring?

Remove the mulch at seed planting time and replace it once the seedlings are established and beyond the delicate phase of sow bug damage.

What exactly is the mulch for, while growing seedlings? When I overwinter a bed by applying mulch, I dig it all in before I plant.

I was just saying what I have found in practice in my garden over the years, rather than what I have read. The woodlice congregate where there is undecomposed organic material. They do need some moisture, under pots etc, and they also like darkness, but in my experience they are more plentiful in a too dry compost heap then in a damp one.

As to the sowing depth question, the words used were 'plant seeds deeply' and that doesn't tell the OP how deep to plant. The depth needs to be correct for the type of seed and planting deeper than that is counterproductive.

As to planting deeply, I think you have to interpret that as biasing the planting depth a little on the large side. When I'm instructed to plant my peas 1 to 1 1/2 inch deep (which is the usual package instructions), I'm given a range of productive depth. In this case use the deeper one. When the instructions say to plant 1 inch deep, I'm not going to be out there with a micrometer getting it exact. If I want my plants to emerge more maturely, I'll plant them a little deeper. No sweat.

As to moisture and woodlice, I've never seen them in dry environments. They are crustaceans that otherwise are wholly aquatic. Their shells are quite permeable, and not waxy, so they lose water rapidly in warm environments. They actually breathe through a gill-like mechanism - not lungs, and moisture is extremely important for that. That being said, I believe there are some rarer species that actually do like it dry.

Other than as a danger to very small seedlings, pillbugs are a good guy. They can even remove heavy metals from soil. Fun article - since it's a school garden you might want to share some of this. http://naturalon.com/healthy-facts-about-those-roly-poly-bugs-in-your-yard/#ixzz4leYTM8lo If you have that many - they may be doing all the damage, but birds also love and make off with many small seedlings and slugs, snails, earwigs, flea beetles and grasshoppers will also wipe them out.