Nursery and Landscape - Insecticide Phytotoxicity

Some insecticides, expecially those that are in emulsifiable concentrate form, can cause plant injury if used in hot weather. Some insecticides also interact with specific plant varieties to cause damage. The following is a good article on insecticide phytotoxicity.

Phytotoxicity is general term used to indicate spray injury to plants. The signs (chlorosis, burning, tissue distortion, or plant death) may appear from within a few days of application to several weeks later. This article only considers phytotoxicity due to insecticide applications but injury can result from application of other pesticides, nutrients, or other substances. Some plant species or varieties are sensitive to particular chemicals, in other cases, injury occurs only under certain environmental conditions.

Here is an overview of phytotoxicity base on major contributing factors.

1) Some species or cultivars are highly sensitive to particular active ingredients, or inert ingredients in some formulations.

2) Applying more than the labeled rate at one time or treating at very close intervals.

3) Tank mixing several pesticides. Synergism or antagonism from combinations in spray tanks can result in injury. Some formulations contain ingredients that interact with ingredients in other products.

4) Pesticide applied at a susceptible growth stage - around bud break or during flower development.

5) Stressful environmental conditions - especially during periods of excessively high temperatures, while plants are under drought stress, or when sensitive foliage is wet longer than normal.

Reducing phytotoxicity

Be aware of susceptible species or cultivars for the products you use.

Use separate tanks / sprayers for herbicides, insecticides, fungicides as practical.
Read the label before application - especially check for instructions on tank mixes and potential for plant injury. Follow application rates and retreatment intervals.
Avoid adding adjuvants (spreaders, stickers, wetting agents, etc.) unless their use is specifically stated on the label.

Treat small numbers of plants with tank mixes and watch for symptoms before treating large areas or numbers of plants.

Don't treat under adverse environmental conditions (temperature, humidity) or when plants are under stress.

Select formulations carefully. Solvents in emulsifiable concentrates (EC) formulations tend to have a much higher potential for damaging plant tissue than wettable powder (WP) or flowable (F) formulations.

Be aware of pest damage or abiotic conditions that can cause injury that resembles phytotoxicity. Patterns and timing or appearance of symptoms may be helpful in identifying potential causes of injury.

In some cases, pH of water used in the spray tank may affect pesticide performance.
Example label statements for selected insecticides

Horticultural oil - Do not apply during periods of drought or when plants exhibit moisture stress. Injury may occur on the most oil tolerant plants if relative humidity is low and wind and temperature are high. Summer spray oils should not be applied when the temperature is 90F or above or when the humidity is high. Blue spruce may lose its blue color for a few years after an oil spray.

Insecticidal Soap - Do not spray when plants are under stress. Avoid spraying during full sun. Spray early in the morning or evening, or when overcast. Soap spray may cause marking of some varieties.

Malathion - Injury may occur on certain ferns including Boston, Maidenhair, and Pteris, as well as some species of Crassula. Before using Malathion EC alone or with any other material, make a test application on a few plants and observe for 7 to 10 days prior to treating large areas to reduce the possibility of plant injury.

Orthene 75% Turf Tree, and Ornamental Spray (acephate) - Do not apply to Huckleberry, Balm of Gilead, cottonwood, Lombardy poplar, and Viburnum suspensum.
Phytotoxicity has occurred on the following Crabapple varieties: Hopa, Ichonoski, Malus floribunda, Pink Perfection, Red Wine and Snow Cloud. Phytotoxicity has occurred on Bletchum gibbum, Cissus antarctica, Ficus triangularis, Fittonia verschaffeltii, Maranta leuconeura kerchoveana, Pachystachya lutea,, Plectranthus australis, Polypodium aureus, Polystichum, Pteris ensiformis, Tolmiea menziesii. A spray on Poinsettia after bract formation may result in phytotoxicity on certain varieties. Phytotoxicity has occurred on the following Chrysanthemum varieties: Albatross, Bonnie Jean, Dixie, Garland, Gem, Iceberg, Pride, Showoff, Statesman, Tally Ho, Westward Ho, and Wild Honey. Do not apply to Chrysanthemums and Roses with open flowers. Tank mixing or use of this product with any other product which is not specifically and expressly authorized by the label shall be the exclusive risk of the user, applicator, applicator/advisor.

Sevin (carbaryl) Application to wet foliage or during periods of high humidity may cause injury to tender foliage. Do not use on Boston ivy, Virginia creeper, and maidenhair fern as injury may result. Carbaryl may also injure Virginia and sand pines. The use of adjuvants may increase the potential for crop injury to sensitive crops. General statement - It is impossible to eliminate all risks associated with the use of as product, Crop injury, ineffectiveness, or other unintended consequences may result because of such factors as weather conditions, presence of other materials or the manner of user application, all of which are beyond the control of the manufacturer. All such risks shall be assumed by the user or buyer.

Reprinted from "INSECTICIDE PHYTOTOXICITY" By Lee Townsend in the June 9, 2008 edition of the Kentucky Pest News.