Lesser bulb fly

Leo A. Martin leo@possi.org
Thu, 15 Nov 2012 10:08:11 PST
Angelo writes regarding bulb flies

> causing me several losses of big Brunsvigia, Amaryllis belladonna
> Multiflora, Veltheimia capensis. It seems to like the bigger bulbs,
> partially emerging from the soil. The imidacloprid is not effective so
> far, I have spried the plants while in leaf past season.

I don't have bulb flies but here we have a native weevil that attacks
Agave species in the same manner. It prefers expensive and exotic species,
which will not surprise anybody here. If you drink Mexican mezcal with a
grub in the bottle you will see the larva of this weevil.

The insecticide has to be where the female enters the plant so she is
killed before she can lay eggs. If the bulb fly enters through the crown
you would need to keep granules or drench there, and you would have to
retreat after rain or overhead watering. Our weevil enters where the stem
touches the soil.

We use permethrins or pyrethrum as a drench on the soil. Instead of a
permethrin drench, some people use imidacloprid granules sprinkled around
the plant and into the leaves. Overhead watering leaches imidacloprid from
the granules to the soil around the stem. When we plant a new Agave we put
imidacloprid granules into the soil where we expect the plant to extend
its roots.

With the drench plants need to be retreated after rain, and the
imidacloprid is destroyed by sunlight, so the granules need to be replaced
regularly. We treat our agaves monthly March through September plus after
each rain. If we put imidacloprid in the soil at planting time we don't
need to treat any more.

Leo Martin
Phoenix Arizona USA

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