Maintaining Water Features without Harboring Mosquitoes and

Maintaining Water Features without Harboring Mosquitoes and Black Flies
The trend has been to create outdoor living space that is lush, cool, and takes you “aware from it all”.
Because of the protracted drought in California some residents are beginning to move to a water-wise
landscape. The new water-wise design may or may not include a water feature. If you wish to retain or
install a water feature, whether it is a pond, fountain or similar water-holding focal point, there are
some things to consider.
Water-loving Pests:
There are several species of flies that are prone to
enjoy your water feature just as much as you do
and turn your sanctuary into a real nightmare.
Mosquitoes and black flies both depend on water
to complete their life cycle and quickly emerge as
adults to reproduce. Both types bite and
mosquitoes can transmit disease to people and
pets.
A Mosquito’s Basic Needs:
The mosquito’s life cycle is dependent on
the presence of standing (trapped or
stagnant) water. Females deposit eggs
either on the surface or around the edges
of water just at edge. Some mosquito
species lay eggs individually, but our most
common urban mosquitoes (Culex) lay 150200 eggs at once in rafts which float on the
surface. Mosquito eggs take
approximately two days to hatch; the
worm-like larvae hatch into the water and
hang head-down while they feed on
bacteria and algae gleaned from the water.
When disturbed, mosquito larvae wiggle
down and hide in the bottom debris. After
a minute or two they will slowly rise to the
surface again. The larvae rest at the
surface and draw air through a siphon at the end of their bodies. For this reason they require still water
to maintain contact with the surface to breathe. The larvae molt multiple times and then pupate. At
this stage they stop feeding. Mosquito pupae resemble tiny shrimp and tumble about when disturbed.
The adult mosquito emerges from its pupa case after a few days. The complete life cycle averages only
7 days in warm summer months and varies slightly depending on species and temperature.
Black Flies or Buffalo gnats (Standing Mosquito Habitat Management on its Head):
Black flies are small gnat sized biting flies measuring only 1/8 inch or so. The larvae are very tiny and
look like black grubs or worms adhered in groups or large clusters to underwater surfaces where water
is flowing rapidly. Females bite to obtain blood for egg development like mosquitoes, but unlike
mosquitoes, males also bite and feed on blood. Female black flies will dive under water and attach their
eggs to surfaces where water is moving and well oxygenated. Once hatched, the larvae can move
around and may work their way into pond or fountain filters and pumps.
Pond Design and Maintenance with Pest Control in Mind:
Mosquitoes can develop in
water features with filters,
fountains and/or falls and in
the presence of predators if
places exist in the feature
where water is trapped. This
can happen in very shallow
areas of the water feature, in
areas where water is stilled by
overgrown plants, or dense
stems, leaves or root systems,
or by aquatic plant pots placed
in the water where the edges
of the pot rise above the water
line trapping water inside the
pot and blocking access by predators such as mosquitofish. Design is key and the deeper the pond the
better. Predatory fish need a deep section to escape their own predators (birds and raccoons –
primarily) and to survive cold winter months. The edges of the pond should be steep (more than a fortyfive degree angle (90 degree is best) to eliminate any possible hiding places for insect larvae. Shallow
sloped edges create warm water sanctuaries and allow larvae to develop out of reach of mosquitoeating fish and other potential predators. Also be aware of what is planted around the outside edge of
the pond that may overhang into the water and provide hiding places for developing mosquito larvae.
Potted plants placed into the pond can become problematic if the sides of the pot are exposed above
the surface of the water preventing water movement and blocking predatory fish.
Trapped water
Water level in partially submerged pot provides a place for mosquito larvae to develop. Be sure to add
enough substrate to raise level above-water level in pot to eliminate standing water inside the pot.
Egg rafts
Egg rafts
When the edge of the pot traps water and creates a barrier to predators and water movement
mosquitoes can proliferate.
Aquatic plants will help keep water cool, and provide hiding places for young mosquito fish (from their
predatory parents and others). But, too many plants choke the water surface and make it difficult for
fish to find mosquito larvae. Certain types of water plants with massive amounts of finely divided root
hairs (such as water hyacinths) can create hiding places for mosquito larvae and should be avoided.
Water Movement (Timing is everything!):
For mosquito control: Add surface agitation. This can be done
with a waterfall or fountain. If the surface of the water is
constantly moving this action prevents larvae and pupae from
breaking surface of the water to breath. They must be able to
engage the surface tension to “stick” to the surface of the water
(they don’t have gills or any means to extract oxygen from the
water column like fish and dragonfly larvae do). If the water is
moving, the current acting on the weight of their body pulls
them away from the surface and they must struggle to keep
their siphon (breathing tube) above water. The presence of moving water will even discourage female
mosquitoes from laying eggs in a water feature. Anything added to the pond that creates hidden, stillwater areas in a pond may still allow mosquito larvae to survive.
For black fly control: Shutting off the flow of water
for 24 hours once a week will rob developing black
fly larvae and pupae of oxygen and they will die. If
you leave the water off for too long (5 days or
more) then you run the risk of harboring mosquito
larvae. Five days may just be enough time for eggs
laid on the first night the flow is shut off to hatch
and emerge into biting adults if the nights are nice
and warm (insect growth rates are most rapid
when temperatures are at their highest).
Other Control Methods: The use of biological
predators such as mosquitofish is quite effective
as long as your pond is not too choked with plants or other barriers to the fish. Don’t forget natural
predators. Not all bugs are pests and many aquatic species are predators of mosquitoes. Don’t
discourage water striders, backswimmers, dragon and damselfly larvae are natural predators of juvenile
mosquitoes. Be sure to maintain plantings both in and around your water feature to avoid creating
protected pockets for pests. There are also a number of products on the market which will help with the
control of the level of bacteria and algae in a water feature and help
to create an environment that discourages mosquitoes from
depositing their eggs. There are products available which kill aquatic
fly larvae with pathogenic bacterial additives or with an insect growth
regulator (IGR). We have found that these products are difficult to
use successfully by many homeowners and we don’t encourage
reliance on those products. Careful design of a water feature to avoid
creating safe harborage for mosquito larvae is most important.