One common question asked by greenhouse producers is associated with the effectiveness of systemic insecticides against the western flower thrips, Frankliniella occidentalis. Western flower thrips (WFT) are the most important insect pest of horticultural greenhouse-grown crops worldwide. In order to develop controls, it is essential to understand the feeding behavior of these pests.
WFT have piercing-sucking mouthparts, but they do not feed exclusively in the phloem sieve tubes. Instead, they feed within the mesophyll and epidermal cells of leaf tissues. More specifically, they feed on plants by inserting their tubular stylets into cells and withdrawing the cellular contents. This feeding behavior may inhibit the effectiveness of systemic insecticides against WFT; however, this is dependent on whether they are feeding on leaves or flowers. In addition, the anthophilic (inhabiting flowers) nature of WFT limits their exposure to systemic insecticides for several reasons:
The active ingredient is not readily transported into flower tissues (petals and sepals).
The concentration of active ingredient that is translocated into flower parts may not be sufficient to directly kill the thrips.
The active ingredient of a systemic insecticide may degrade faster in flower parts and differences in the transpiration rates between flowers and leaves may result in flowers being less efficient sinks for the active ingredient of systemic insecticides.
Flowers don’t last as long as leaves, so there is less time for systemic insecticides to accumulate compared to the foliage.
Systemic insecticides may not provide fast knockdown to prevent thrips damage to flowers when abundant populations are present.
All of these factors, however, may depend on the systemic insecticide and the associated water solubility, because systemic insecticides with greater water solubility may accumulate in flower parts at concentrations sufficient to kill WFT.
Water Solubility Is Key To Efficacy
Systemic insecticides applied to the soil/growing medium must be water-soluble to some degree in order to allow the dissolved active ingredient to be absorbed by plant roots. Water solubility determines how rapidly the active ingredient is absorbed by roots and translocated throughout plant parts such as leaves and stems. A highly water-soluble systemic insecticide may kill insect pests quickly; however, it may not provide long-term or sufficient residual activity compared to a less water-soluble systemic insecticide. A less water-soluble systemic may persist longer, but may not be as effective unless the rate is adjusted to compensate for the slower mobility.
Table 1 presents the systemic insecticides labeled for use in greenhouse production systems that can be applied to the soil/growing medium, and their corresponding water solubilities.
|Table 1. Insecticides with systemic activity, when applied as a drench or granule to soil/growing medium that are commercially available for use in greenhouse production systems including common name (=active ingredient), trade name, and corresponding water solubility.|
|Common Name||Trade Name||Water Solubility (ppm)|
|Azadirachtin||Azatrol, Aza-Direct and Molt-X||0.50|
|**Acetamiprid (TriStar) is not labeled for soil/growing medium applications. It is only registered for use as foliar or sprench applications. This is why acetamiprid is not included in Table 1.|
Here’s one example of how water solubility influences the uptake and efficacy of systemic insecticides. Imidacloprid (Marathon), which has a water solubility of 0.51 g/L or 500 ppm, tends to be less effective against flower- and pollen-feeding insect pests including WFT. Research has shown that acephate, which has a water solubility of 790 g/L or approximately 79,000 ppm, is converted into the metabolite — methamidiphos and actually moves into flowers, protecting them from WFT feeding injury. It may provide systemic protection to flower buds, which allows plants to flower and minimizes feeding injury resulting in good flower quality.
WFT feeding on leaves (both nymphs and adults) tend to be more susceptible to systemic insecticides than when feeding in flowers. Leaf-feeding more easily results in the insects imbibing toxic concentrations of the active ingredient of systemic insecticides. For example, it has been reported that WFT feeding on plant leaves are “suppressed” by thiamethoxam (Flagship) when applied to the soil/growing medium. The water solubility of thiamethoxam is 4.1 g/L or 4100 ppm. However, it is possible that the metabolite — clothianidin — is actually responsible for killing the thrips. Although the water solubility of clothianidin is 0.32 g/L or 327 ppm, the material translocates throughout the entire leaf, potentially exposing thrips to lethal concentrations of the active ingredient.
Systemics Can Be Used As A Contact Spray
Spray applications of systemic insecticides tend to be more effective than soil/growing medium applications because they are being primarily used as contact or translaminar sprays, and not so much for any systemic activity. For example, sprays of acetamiprid (TriStar)* and thiamethoxam (Flagship) have been shown to be effective against WFT nymphs and adults. In our research efficacy trials, we have found that the systemic insecticide dinotefuran (Safari) provides sufficient (greater than 80 percent) mortality of WFT when applied as a foliar spray.
In summary, due to the feeding behavior of the WFT, systemic insecticides, when applied to the soil/growing medium, in general, may be less effective than when applied as foliar sprays. Therefore, it is important to understand that when using systemic insecticides for regulation of xylem- and phloem-feeding insect pests, the use of spray applications of contact or translaminar insecticides will be required to regulate populations of the western flower thrips.