PHOTOS BY JOHN FECH, UNL.
Damage to the cambium can be responsible for reduced pesticide uptake.

Pesticides don’t always work. Sometimes they work great, and at other times, they deliver some level of performance, but certainly not up to expectations. Occasionally, they don’t work at all.

For the most part, pesticides produce reliable results. In addition to the safety requirements the EPA demands, the manufacturer itself puts potential products through rounds of performance testing to make sure the product will produce a reliable outcome for the end user.

Overall, keep in mind this key point: Regardless if it is designed to control insects or diseases, it is rare that the pesticide itself is the cause of the failure. In almost every case, it’s due to one of many factors that decrease its performance. Let’s take a look at possible causes for pesticide failure in the tree care industry.

Timing is crucial for control of pests. A fungicide to control apple scab is not likely to a prevent significant level of infection if applied at this stage.

Possible causes for pesticide failure

• Timing-It’s all about timing. Unlike the turf care market where a timing error often leads to a temporary, six to eight-week period of time with a dead lawn, improper timing in tree care frequently leads to a situation where the tree has only dead leaves or needles until the next spring when it produces new growth. Each pest has a window of vulnerability. For many insect species, it’s the life cycle stage that is least protected, such as a larvae or nymph. For many foliage diseases, it’s in early spring at bud break when new leaves are forming. Treatments made after these time periods offer much reduced efficacy.

• Drift-Drift is problematic on several fronts. First, it lowers the effectiveness of the application, and leads to what we think of as pesticide failure. If 40 percent of the applied spray formulation fails to contact the tree trunk or canopy due to drift, then it’s reasonable to expect that at least 40 percent fewer pests will be controlled or prevented than expected.

Secondly, when excessive wind causes drift, damage to nearby plants, cars and other valuable objects can occur. Third, there’s the cost factor. If a portion of the applied pesticide fails to contact the plant surface, the applicator tends to keep spraying until the correct amount hits the leaf or trunk, which means that more volume than is necessary is applied. The cost of overapplication is simple: more product, more cost.

Timing is crucial for control of pests. A fungicide to control apple scab is not likely to a prevent significant level of infection if applied at this stage.

• Mixing errors-When the wrong rate, wrong product or wrong pH water is part of the tank mix, the pesticide application is likely to fail. Certain pesticides, especially fungicides and insecticides, rely on a neutral pH to be effective. If a product is mixed with 8.1 pH water instead of the preferred 6.1 pH, the effective longevity can be reduced from weeks to days or even just a few hours. The label on the pesticide container will indicate if this is a concern, indicating the desired pH range for water in the spray tank. To determine the pH of your water source, start with the laboratory that tests soils in your area.

• Product degredation-Pesticide products can lose their effectiveness over time, especially if they are stored differently than what is recommended on the label. Generally, liquid products must be stored between 40 and 100 degrees in order to maintain product stability. To avoid excessive degredation, try to purchase in relation to what you expect to apply throughout the season. Products can be stored through the winter and used the next year, but unlike wine, they don’t get better with time.

• Untimely weather events-Heavy rainfalls or drastic temperature changes that occur after a pesticide application can cause ineffectiveness. Fortunately, advances in technology in weather forecasting make it easier to choose the best days for applying pesticides.

• Tree inadequacy-Pesticides that rely on the plant system for uptake can be ineffective if the plant itself is weak. Systemic insecticides, such as imidacloprid, rely on strong, active roots for uptake, as well as a healthy cambium layer to move the product throughout the tree. Because weak roots don’t absorb well, and damaged tree trunks don’t contain viable cambium, incomplete control can occur. Tree root systems in urban areas, such as parking lots and street tree pits, are commonly compromised, as well.

• Wrong diagnosis-This occurs occasionally, especially with new hires or with technicians without much experience in distinguishing between pest problems and cultural disorders. If a client’s tree was diagnosed with problem A and the actual cause is B, then the pest control agent selected for A is unlikely to work well for B. Different disease and insect pests have different behaviors, life cycles and susceptibilities to pesticides and other control measures. Also, because many similarities exist between pest problems and look-alike pest problems, it’s crucial to accurately identify the target pest problem.

• Sprayer calibration-If it’s been a year or so since your spray equipment has been adjusted, it might be time to do so. Improperly adjusted sprayers can be the cause of failure, with too much or too little active ingredient being applied.

• Inadequate coverage of bark, leaf or trunk-A poorly calibrated sprayer is one cause of inadequate coverage, but the other is simple applicator error. A spray pattern indicator can help newly hired technicians learn what to expect in terms of distribution from various nozzles.

• Unrealistic expectations-This last possibility is characterized by unrealistic expectations, either by the technician or the customer. A good example is a treatment made to kill aphids on a maple tree. Sure, the treatment was made, the pests died and discontinued feeding, but the problem is that the brown leaves that the aphids caused won’t turn back to green after they’re dead. The solution is to learn how to explain to the client exactly what to expect from a potential pesticide application, as well as what not to expect.

It only takes one of the above problems to cause a pesticide failure or reduced efficacy. In some cases, two or more factors lead to less than desirable results. Make sure each factor is considered every time a pesticide application is made.

The author is a horticulturist and certified arborist located in Omaha, Neb.