Pesticides are substances designed to prevent, destroy, repel, or mitigate pests, such as insects, weeds, fungi, or rodents. These chemical tools are used in agriculture and horticulture to protect crops and ensure production yields. Systemic pesticides represent a specific category defined by their unique ability to enter and circulate throughout the plant’s internal structure, distinguishing them from treatments that remain only on the surface.
Defining Systemic Action
A systemic pesticide is a chemical compound that a plant absorbs and translocates throughout its vascular system, distributing the active ingredient to various tissues. The pesticide is taken up through the leaves, roots, or seeds, making the plant toxic to feeding pests. Systemic action is utilized across insecticides, fungicides, and herbicides.
This internal protection contrasts with contact pesticides, which only control pests physically touched by the chemical residue. Contact materials form a protective layer on the exterior surface, meaning their effectiveness relies on thorough coverage and can be lost due to rain or irrigation.
Systemic compounds provide sustained control by protecting newly developed tissues, such as young leaves and shoots, which were not present at the time of application. This ability to protect hidden or hard-to-reach parts of the plant, like the root system or the underside of leaves, is why they are widely used against sap-sucking insects such as aphids and mealybugs.
Internal Movement and Distribution
Once a systemic pesticide is absorbed, its movement within the plant is called translocation, managed by the plant’s two primary vascular systems.
The first is the xylem, which transports water and dissolved minerals from the roots upward. Pesticides that are highly water-soluble are often mobile in the xylem, moving in a unidirectional flow toward the leaves and growing tips.
The second pathway is the phloem, which transports sugars and nutrients throughout the plant, including downward to the roots. Some systemic compounds are designed to be phloem-mobile, allowing them to move bi-directionally. This movement is highly effective for distributing the chemical to all parts of the plant, including the roots, fruits, and newly formed tissues.
The pesticide’s chemical properties, particularly its water solubility, determine whether it moves primarily through the xylem, the phloem, or both. Xylem-mobile compounds protect the upper canopy following a soil application, while phloem-mobile chemicals ensure a more complete distribution. The speed of transport varies, often being slower in woody plants, meaning control can take days to several weeks.
Common Application Methods
Systemic pesticides are applied using methods designed to maximize the chemical’s uptake by the plant.
One common technique is seed treatment, where the pesticide is coated onto the seed before planting. As the seed germinates, the chemical is absorbed by the developing roots and distributed throughout the seedling, providing protection from the earliest stages of growth. This method also minimizes the amount of active ingredient released into the environment.
Another major method involves applying the chemical to the soil, often as a soil drench or in a granular form, allowing the roots to absorb the active ingredient. For a soil drench, the product is mixed with water and poured around the base of the plant. For large trees, a trunk injection can deliver the pesticide directly into the xylem tissue, ensuring rapid distribution within the canopy.
Foliar sprays are also used, requiring the chemical to penetrate the leaf surface and be absorbed into the internal tissues. The effectiveness of foliar application depends on the chemical’s ability to pass through the waxy cuticle. Once absorbed, the chemical is protected from environmental factors like rainfall and sunlight, providing persistent control.
Persistence and Non-Target Organism Exposure
The internal and persistent presence of systemic pesticides has significant implications for environmental exposure and food safety. Once absorbed, the chemical is protected from surface wash-off and environmental degradation, allowing it to remain active within plant tissues for extended periods. This long-lasting activity is beneficial for pest control but means the active ingredient can be present throughout the plant’s life cycle.
A major concern stemming from this persistence is the exposure of non-target organisms, particularly pollinators like bees and butterflies. Systemic insecticides can be translocated into the plant’s reproductive structures, contaminating pollen and nectar consumed by foraging insects. Residues detected in these floral resources can cause lethal or sublethal effects on pollinators, contributing to declines in beneficial insect populations.
Furthermore, the systemic nature means residues are present inside harvested plant material, including fruits, vegetables, and grains. Because the chemical is integrated into the plant’s structure, it cannot be removed by washing or peeling. This internal presence results in consumer exposure through the diet. The persistence of the active ingredient is a primary consideration in assessing both ecological and food safety risks.