Plants produce chemicals that function as defensive mechanisms to reduce injury from plant-feeding insects and mites. Mixtures of these chemicals, referred to as plant-derived essential oils, are primarily isolated during the process of steam distillation of aboveground plant parts. A number of these chemicals posses both insecticidal and miticidal properties with a broad-spectrum of activity including antifeedant and repellent activity, molting and respiration inhibition, growth and fecundity reduction, cuticle disruption, and they are active on the central nervous system. The advantage of insecticides and/or miticides with multiple modes of action is a potential delay in resistance development. Plant-derived essential oils, used as insecticides or miticides, typically have short residual activity with restricted entry intervals of less than 12 hours because they are susceptible to temperature and light degradation. This allows growers to enter treated areas within a short period of time, but this short residual activity may mean that repeat applications are required, possibly resulting in plant injury or phytotoxicity.

Essential oils derived from rosemary (Rosmarinus officinalis), peppermint (Mentha piperita), sweet marjoram (Majorana hortensis), sweet basil (Ocimum basilicum), mint (Mentha viridis), and lavender (Lavandula officinalis) have demonstrated effectiveness against different species of spider mites including the twospotted spider mite (Tetranychus urticae), the melon/cotton aphid (Aphis gossypii) and green peach aphid (Myzus persicae).

Despite the efficacy of plant-derived essential oils against certain arthropod pests, one of the primary concerns associated with their use is the possibility for plant injury or phytotoxicity. In fact, many plant-derived essential oils have been shown to be phytotoxic to vegetables, and herbaceous and foliage (tropical) plant material. Nonetheless, the extent of any plant injury or phytotoxicity may be dependent on the concentration or rate applied, plant type, and plant parts (foliage vs. flowers) exposed during spray applications.

Many plant-derived essential oils are considered minimum risk pesticides and are exempt from Environmental Protection Agency (EPA) registration under section 25(b) of the Federal Insecticide Fungicide and Rodenticide Act (FIFRA). These  active ingredients  are exempt: castor oil, cedar oil, cinnamon oil, citronella oil, clove oil, cottonseed oil, garlic oil, geranium oil, lauryl sulfate, lemon grass oil, linseed oil, mint oil, peppermint oil, sesame oil, sodium lauryl sulfate, soybean oil, thyme oil

One of the major factors that may impact the use of plant-derived essential oils in crop production systems is lack of consistent control. The reasons for inconsistent control of arthropod pests associated with plant-derived essential oil products include 1) time of year or season plants are harvested; 2) geographic location plants are produced; 3) temperature and light exposure plants have received; 4) plant nutritional quality; 5) plant parts harvested; 6) environmental “stress” plants have been exposed too; 7) plant production practices; 8) product formulation; 9) concentration of active ingredient; and 10) availability of plant resources. Additional concerns associated with plant-derived essential oils that need to be taken into consideration are 1) phytotoxicity, which may be due to the inerts in the formulation such as solvents or carriers, and not the active ingredient; 2) variability obtained from both laboratory and field results; and 3) allergic reactions by workers or customers.

Based on our research at Kansas State University (Manhattan, KS) and the University of Illinois (Urbana, IL), we have found that many of the plant-derived essential oils available to both professionals and homeowners (consumers) are ineffective in managing the populations of certain arthropod pests including western flower thrips (Frankliniella occidentalis), green peach aphid, twospotted spider mite, sweet potato whitefly B-biotype (Bemisia tabaci), citrus mealybug (Planococcus citri), and longtailed mealybug (Pseudococcus longispinus). Furthermore, we have experienced problems associated with phytotoxicity to both foliage and flowers.

The lack of efficacy against certain arthropod pests may be associated with minimal persistence, due to volatility, and short residual activity of the essential oil, indicating that it may take longer to kill arthropod pests with products derived from essential oils compared to the commonly used pest control materials. In addition, differences in formulation may affect the ability of plant-derived essential oil products to provide both short and long-term control of arthropod pests.
In conclusion, pest control materials (insecticides and miticides) derived from plant essential oils continue to enter the market for use in crop production systems. However, growers need to exercise caution due to the concerns associated with phytotoxicity. The use of plant-derived essential oils may become feasible after we understand how to expand their effectiveness and develop formulations (e.g. micro-encapsulation) that alleviate problems associated with phytotoxicity.  

Dr. Raymond A. Cloyd is Extension Specialist in Ornamental Entomology/Integrated Pest Management  at Kansas State University. He can be reached at rcloyd@ksu.edu.