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 In
addition to water lilies, this aphid feeds on many other
species. It is
also known as the “reddish-brown plum aphid”, a name that
is derived
from its association with fruit trees, particularly during
winter. This insect is widely distributed
(cosmopolitan), and has
long been known as a pest of cultivated aquatic
plants. Aphids
suck sap from plant leaves, but can also cause damage by
transmitting
plant viruses. The waterlily aphid is extremely
destructive in
aquatic gardens and nurseries and is known to transmit at
least five
plant viruses.
Winged adult females migrate from aquatic habitats to
fruit
trees in late fall and lay their eggs on the trees.
There they
overwinter, and, after hatching, subsequent generations
spend the
spring
and early summer on the fruit trees.
In mid- to late summer, the aphids migrate to aquatic
plants. While on
fruit trees, they are oviparous (egg laying) but after
migrating to
hydrophytes, they become ovoviviparous (retaining their
eggs in their
oviducts until the eggs hatch and then giving birth to
living young).
The ovoviviparous females are wingless, but those that
migrate to and
from fruit trees are winged. The waterlily aphid can
walk on the
water
surface and can crawl down under fronds to feed
underwater.
Specialized “hairs” on their bodies trap and hold air
while the aphids
are underwater.
After colonizing aquatic sites, the aphids reproduce
quickly. The
developmental period from the birth of the first instar to
the adult
stage ranges from 7 to 10 days, depending upon temperature
(optimally
21-27°C). Each female produces up to 50 nymphs at an
average rate
of two to four nymphs per day. The nymphs normally
progress through
five instars during the course of their development,
although they will
occasionally produce a sixth instar.
Photos above: A, Adult female and
two
nymphs of the waterlily aphid. B, Relatively
dense
waterlily aphid colony on the
underside of a waterlettuce leaf.
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In nature
duckweeds serve as a
food source for a wide variety of
animals and microorganisms. These range from familiar birds and
fish that may
consume
duckweeds as part of their diet to little-known
insects and microorganisms
with specialized dependency on these
plants. Aquaculture systems have been designed in which
domesticated fowl or fish are raised on duckweed grown in managed
ponds. In other cases, herbivorous fish or insects are
used
to control unwanted growth of duckweeds. This page provides
information on just a few of the organisms that feed on duckweeds
in
nature.
The
duckweed fly grows primarily on duckweeds and is one of
the smallest
insects to attack an aquatic
plant. 
The
duckweed
weevil Tanysphyrus lemnae is one of
the most common and widespread duckweed herbivores.
Pythium
myriotylum is one of the
most common
species of Pythium
found in
the soil in damp climates,
often
causing damping-off of seedlings and root-rot, so this
species cannot
be said have any special affinity for duckweeds.
These
fungi produce masses of microscopic, motile zoospores (see
drawing) that can swim short distances to attack wet
surfaces of
plants. The fungi produce enzymes that break down the
pectin in
plant cell walls. Pectin breakdown results in a soft,
watery
rot. Pythium can survive indefinitely in the
soil as a
saprophyte, feeding on soil organic matter. In the
soil it can
also form thick-walled sexual oospores (see drawing).
Oospores are the primary overwintering form. Pythium
species are not vigorous competitors with other
microorganisms in the
soil. The fungi are disseminated in surface-drainage
water and in
infested soil on farm equipment, tools, and the feet of
humans and
animals.