Q. I grow a lot of aquatic plants in my greenhouse throughout the year. They vary from submerged oxygenating plants to any type of water garden plant possible. My problem is with aphids on my water lettuce, salvinia and azolla. These black and sometimes greenish-brown intruders will usually cling to the leaves of these plants, feed off them, and then eventually cause them to die after months.
Is there any way to control these pests? Everyone I have asked says I should just use a high-pressure water sprayer to blast them off the leaves. Normal hose spray does not seem to work because somehow the aphids cling to the leaves. I would appreciate any help you can offer.
A. The pesky black bugs, which are well known to every water gardener, are indeed aphids — they are called blackfly aphids. These insects are easily transported on aquatic plants.
A few of these aphids arriving with newly purchased plants are essentially impossible to spot. They reproduce rapidly and quickly become a nuisance. If allowed to get out of control they will kill plants by sapping them of vital juices and then allowing bacterial and fungal invaders to enter the plant. Once they become established in a water garden they are virtually impossible to eradicate. They can, however, be controlled, and once controlled they pose no danger to plants.
First, these particular aphids apparently spend the cooler winter months on cherry trees, plum trees or other fruit trees in the vicinity. So, if you have some of these trees in or around your greenhouse you might consider using an oil spray over the winter to reduce the aphid population.
Second, if your aquatic plants are not part of a pond or other water feature harboring animals — especially fish, amphibians, reptiles or invertebrates — then you could use any one of a number of commercial insecticides, such as malathion. But think real hard about this option. Even the casual use of products like malathion can decimate local fauna. For example, any toads or tree frogs that might be in the vicinity of your greenhouse will most likely be wiped out after a few treatments. You will also kill off beneficial pollinating insects.
Increasingly, studies are showing that the larger effect of harming lots of other critters (unintended victims) will probably overshadow the effectiveness of using insecticides against the intended target — in this case, the aphids. The control of the aphid population and their detrimental effect on your plants can be accomplished in a more benign way.
I would suggest you stick with the tried and true high pressure water spray method. An ordinary adjustable nozzle for a standard garden hose should work just fine. Blast the bugs twice a day for a few days. Repeat the process every week and you should notice a reduction in both the presence of the aphids and the damage they cause. Small fish will eat aphids knocked into the water by the spray.
There are some non-toxic soap and oil sprays on the market that are very good in controlling aphids, and they can enhance the effect of the power wash. When used properly these will not harm non-aquatic critters in the vicinity. Check with local garden centers or mail order firms specializing in natural gardening. But again, do not use these if you have aquatic animals in the waters supporting your plants.