Crane fly INFESTATION!
I've never seen this in my yard, but this March there are TONS of crane flies living in my lawn! I can't walk out back through the grass without scads of them coming up out of the lawn (EW!). Beneficial nematodes (Steinernema feltiae) is said to help but only about 50% of the fly larvae will be destroyed. I need more help than that. How do I naturally destroy the vast quantities of these insects and their larvae?
There is no way to "naturally destroy" crane fly larvae without destroying your lawn. You did not mention the condition of your lawn, but healthy lawns can tolerate 25 - 50 crane fly larvae per square foot without significant damage.
The presence of the adult "flies" is not a problem as they do not cause damage to turf and there is no correlation between the adults and the number of larvae. The adults you are seeing now are the "common" crane fly vs. the "European" crane fly that have been around for decades. The adults only live a few days, but they emerge over a couple of weeks so you may see them for that long. The European Crane fly adults emerge in early September. With both species, the adults emerge, lay eggs, and then die. The larvae hatch, and if the populations are high enough, can damage the turf. The common crane fly have two generations per year, but we rarely see damage from this species.
One way to look at them is "nature's de-thatcher". However, when the populations get too high, they can kill significant portions of your lawn but it is rarely the whole lawn that dies. Again, the adults do not cause damage so trying to kill them is pointless.
The insecticides and other products (including the nematodes) available are for treating the larvae, not the adults. Additionally, I would not recommend applying an insecticide at this point because the European crane fly larvae are near the end of their feeding cycle and since you did not mention that your lawn was damaged, is not necessary.
Additionally, the populations of crane fly larvae vary considerably from year to year, so the fact that you have a lot of larvae in one year does not guarantee that you have a lot in the following year.
If you are concerned that you will have damage, the best approach for the common Crane Fly is to start sampling in June by using a 6" flat shovel. Cut a 6" by 6" inch whole 3 inches deep and look for the larvae. You have to dig through the soil a little to find them and they will be much smaller than what you are seeing now. Take the number you find and multiply by 4. The result is the number of larvae per square foot. If you have more than 25 per square foot, you may consider applying an insecticide. For European Crane Fly, start sampling in December. Some insecticide choices are listed below, if you determine that a treatment is necessary.
As an alternative, you could apply the beneficial nematodes in April or September as a preventative measure. As you mentioned, they are not highly effective, but they can reduce the populations which may be all you need. I have never applied them but my impression is that a 50% reduction would be a best case scenario.
The European crane fly lay their eggs in September and they need moisture to survive so turning off your irrigation in the beginning of September may greatly decrease the populations of the larvae. Obviously, this technique will not work now because of spring rains.
As far as insecticides go, there are a few that you can look at including Talstar PL or GC, Acelepryn G, or Provaunt (a liguid). Talstar is pyrethroid (see: http://npic.orst.edu/factsheets/pyrethrins.pdf). Acelepryn and Provaunt are relatively new and are extremely safe on beneficials and mammals. But most of these products are toxic to either fish or bees or both. If you are trying to kill an insect, by definition it has to have some toxicity or else it would not be effective. You can search online of labels and toxicity information for Acelepryn and Provaunt. I would encourage you to get your information from a University web site or other science based government website.
The granulars would be applied with a drop spreader, but you need to calibrate the rate before applying. The liquids need to be applied with a boom sprayer and I would recommend hiring a licensed pesticide applicator.