_Phorbia ceparum_ (Meig.)
Early in June a somewhat hairy fly, Fig. 9, may be seen flying about,
and depositing its eggs on the leaves of the young onion plants, near
the roots, Fig. 10.
[Illustration: FIG. 9.]
Dr. Fitch describes this fly as follows: "It has a considerable
resemblance to the common house fly, though when the two are placed
side by side, this is observed as being more slender in its form. The
two sexes are readily distinguished from each other by the eyes, which
in the males are close together, and so large as to occupy almost the
whole surface of the head, while in the females they are widely
separated from each other. These flies are of an ash gray color, with
the head silvery, and a rusty black stripe between the eyes, forked at
its hind end. And this species is particularly distinguished by having
a row of black spots along the middle of the abdomen or hind body,
which sometimes run into each other, and then forming a continuous
stripe.
"This row of spots is quite distinct in the male, but in the female is
very faint, or is often wholly imperceptible. This fly measured 0.22
to 0.25 inch in length, the females being usually rather larger than
the males." The eggs are white, smooth, somewhat oval in outline, and
about one twenty-fifth of an inch in length. Usually not more than
half a dozen are laid on a single plant, and the young maggot burrows
downward within the sheath, leaving a streak of pale green to indicate
its path, and making its way into the root, devours all except the
outer skin.
[Illustration: FIG. 10.]
The maggots reach their full growth in about two weeks, when they are
about one-third of an inch long, white and glossy, tapering from the
posterior end to the head, which is armed with a pair of black,
hook-like jaws. The opposite end is cut off obliquely and has eight
tooth-like projections around the edge, and a pair of small brown
tubercles near the middle. Fig. 11 shows the eggs, larva, and pupa,
natural size and enlarged.
[Illustration: FIG. 11.]
They usually leave the onions and transform to pupæ within the ground.
The form of the pupa does not differ very much from the maggot, but
the skin has hardened and changed to a chestnut brown color, and they
remain in this stage about two weeks in the summer, when the perfect
flies emerge. There are successive broods during the season, and the
winter is passed in the pupa stage.
The following remedies have been suggested:
Scattering dry, unleached wood ashes over the plants as soon as they
are up, while they are wet with dew, and continuing this as often as
once a week through the month of June, is said to prevent the deposit
of eggs on the plants.
Planting the onions in a new place as remote as possible from where
they were grown the previous year has been found useful, as the flies
are not supposed to migrate very far.
Pulverized gas lime scattered along between the rows has been useful
in keeping the flies away.
Watering with liquid from pig pens collected in a tank provided for
the purpose, was found by Miss Ormerod to be a better preventive than
the gas lime.
When the onions have been attacked and show it by wilting and changing
color, they should either be taken up with a trowel and burned, or
else a little diluted carbolic acid, or kerosene oil, should be
dropped on the infested plants to run down them and destroy the
maggots in the roots and in the soil around them.
Instead of sowing onion seed in rows, they should be grown in hills,
so that the maggots, which are footless, cannot make their way from
one hill to another.