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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.