DEPARTMENT OF AGRICULTURE


makes their control much more difficult. The old females, before drop-
ping from the root or tuber, turn from a whitish color to brown as their
cuticle is transformed into a thick, protective cover with a variable
number of eggs and larvae contained inside. Females thus transformed 1
are then called "cysts" and may remain in the soil for years, releasing
active larvae over a period of years. This phenomenon not only compli-
cates control of these pests when crop rotation schemes are used but
also when fumigants are used, since these "cysts" are highly resistant to 1
the action of chemicals. We know of no species of this group occurring
in Florida but are convinced that at least one form, Heterodera weissi,
Steiner, 1949 (Fig. 19) may be present. It is found on various species
of knotweeds and is known to us to occur in numerous states east of the
Rocky Mountains, as far south as the Delta section of southern Missouri
and the region of Ridgeville, Georgia. I
 Root-knot nematodes, together with the various species of the sugar-
beet nematode group, represent sedentary root parasites which are all


 Figure 19.-A. Male of Heterodera weissi, the knotweed heterodera, located
under the surface of a root of Polygonum pensylvanicum L. B. Female Heterodera
weissi as it hangs on a root of the same knotweed as above; X 100.