If you’ve gardened for a while, chances are that you’ve heard the term systemic insecticide. When applied to pesticides, the term systemic means that the chemical is soluble enough in water that it can be absorbed by a plant and moved around in its tissues. Movement of systemic insecticides, like all transportable chemicals in the plant, takes place principally in the plant’s vascular system, which includes the phloem and xylem.
Not all chemical compounds are soluble in water. Most chemicals are going to soluble in water to some degree, or soluble in oil to some degree. Solubility is not an either/or thing. Some pesticides are highly soluble in water, some moderately so. Most pesticides have relatively low solubility in water.
What are the pros and cons of a pesticide being highly soluble in water? On the down side, being highly soluble in water means that a pesticide is more likely to be washed off of a plant by rain or irrigation. Also, high water solubility means that a pesticide may be more easily washed into a stream or (especially in places with sandy soils) seep into ground water. On the plus side, water soluble pesticides may be absorbed more easily into a plant, since plants are largely made of water and the sap is mostly water.
Pesticides that can be applied to the soil beneath a plant and transported in the xylem sap tissue can reach pests that are otherwise hard to kill. Many sap feeding insects, like scales, don’t move around much and may be protected by wax, or by the plant itself, from insecticides sprayed on the leaves and stems. These insects do take in lots of plant sap during feeding, however, so a pesticide in the sap can be easily ingested by the pest.
When systemic pesticides are applied to the soil, beneficial insects, birds and even pets and people are much less likely to encounter the pesticide in the form of residues or spray drift. The treatment of some trees (for example the big oak over the swimming pool) would be difficult to do safely without a systemic pesticide option.
Some of the common house and garden insecticides that are systemic include acephate (Orthene®), imidacloprid (Bayer’s Tree & Shrub Insect Control™, Merit®) and dinotefuran (Greenlight Tree and Shrub Insect Control™, Safari®). You should be especially careful when using systemics if you have a shallow water table under sandy soils, or if you are applying the product near streams, lakes or water features. As with all pesticides, it is important to read and follow the label of a systemic pesticide carefully at the time of purchase, before use, and before discarding any leftover containers or product.