Integrating plant essential oils and kaolin for the sustainable management of thrips and tomato spotted wilt on tomato

The purpose of this project was to develop novel sustainable programs to manage thrips and thrips-transmitted Tomato spotted wilt virus (TSWV), one of the most devastating pest complexes affecting tomato and other vegetable crops in the southern USA (Bauske 1998). We seek to integrate the use of two types of naturally occurring crop protectants, kaolin particle film coatings and plant essential oils, into a more environmentally benign and sustainable management programs for tomatoes. Our project to integrate plant essential oils and kaolin particle films for thrips and tomato spotted wilt management will help reduce the use of synthetic pesticides in tomatoes. Consequently, the results will help to enhance worker safety and environmental and food quality, without sacrificing yields and profits. Because thrips and tomato spotted wilt affect numerous vegetable and ornamental crops in addition to tomato, our findings for tomato will have broad applicability to other agricultural systems.

Tomato is the most economically important vegetable crop grown in the southern US, with a farm gate value of more than $1 billion US dollars. The southern US accounts for upwards of 60% of the fresh market production of the entire US (http://tinyurl.com/s5nlz). Tomato is an expensive crop to produce and historically has high chemical inputs (Bloem and Mizell 2004). Pesticides and pesticide application costs comprise approximately one third of total pre-harvest expenses for field grown tomatoes in Florida (Smith and Taylor 2006, University of Florida http://tinyurl.com/pzyy2). Many of the pesticides used in tomato production are restricted use materials and pose certain risks to the environment and human health. Registration for a number of these materials are now subject to cancellation because of the Food Quality and Protection Act of 1996.

Tomato spotted wilt virus is the type species of the genus Tospovirus in the family Bunyaviridae. Tomato spotted wilt disease was originally described in Australia (Brittlebank 1919), with its viral etiology established in 1930 (Samuel et al. 1930). This virus can only be transmitted to plants by adults of certain species of thrips (Sakimura 1962, Wijkamp et al. 1996, Whitfield et al. 2005, Avila et al. 2006), and only if the adult thrips acquire the virus as larvae (Wijkamp et al. 1996, Assis Filho et al. 2004). Despite the nature of this pathosystem, Tomato spotted wilt virus is one of the most polyphagous plant pathogens, with over 1,000 susceptible plant species (Parrella et al. 2003). Over the past two decades, outbreaks of tomato spotted wilt have increased greatly on a number of crops, such that TSWV is now considered one of the most important pathogens globally, with estimated losses exceeding $1 billion dollars annually. As interest in Tospoviruses has increased over the past ten years, at least 17 additional tospovirus species have been described (Whitfield et al. 2005, Campbell et al. http://tinyurl.com/lwlt8). These differ in serology, genome sequence, vector specificity, and natural host range.

With the advent of thrips-transmitted tomato spotted wilt in the southern US during the late 1980’s, tomato growers in the southern USA relied on applications of broad-spectrum insecticides on a near calendar basis. The current standard insecticide regime consists of rotations of spinosad and methamidophos, with up to 12 – 16 applications per season in attempts to control thrips and TSWV. This intensive insecticide regime has been practiced despite research showing that most losses from TSWV are the result of primary infections, which are not effectively controlled by intensive insecticide use (Puche et al. 1995). Not only is this approach costly, but it poses health risks to farm workers, and it is extremely disruptive to IPM programs for other pests.

Therefore, the need for less costly, safer and more sustainable management tactics for thrips and tomato spotted wilt is clear. We have developed successful alternative management tactics for control of thrips and tomato spotted wilt in tomatoes (Momol et al. 2004) and other solanaceous crops (Reitz et al. 2003). These tactics include the use of highly reflective metalized mulches to repel vector thrips, and systemic acquired resistance (SAR) promoting compounds. While these tactics have proven to be successful and have been widely adopted by southern tomato growers, they do have certain inherent drawbacks. For early plantings, black mulch is preferred because it keeps soil temperature warmer. Furthermore, metalized mulches are approximately twice the cost of standard polyethylene mulches and are more difficult to dispose of because of the combination of metal and plastics. SAR compounds are also expensive and are not approved for use in organic production.

In this project, we intend to develop effective means to integrate the use of environmentally friendly and sustainable materials to reduce the incidence of tomato spotted wilt and thrips in tomato. We propose to use environmentally safe plant essential oils as naturally occurring thrips repellents and to augment the repellent/deterrent effects of plant essential oils with kaolin based particle films. The results of this project will enhance worker safety and environmental and food quality, without sacrificing yields and profits.

Many plants produce volatile essential oils that are thought to function as defenses against pathogens and insect herbivores. These naturally occurring substances are known to have a wide range of biological activities, including toxicity and repellency to certain insect pests. Because of these toxic and/or repellent effects, certain essential oils historically have been used as pesticides against stored grain pests and biting flies (Burfield and Reekie 2005, Isman 2006). Recent work has examined the potential of essential oils to protect crops from pests such as thrips, whiteflies, aphids, caterpillars, and spider mites (e.g., Hummelbrunner and Isman 2001, Choi et al. 2003, Chiasson et al. 2004, Calmasur et al. 2006) Still, widespread commercialization of plant essential oils as crop protectants has lagged, often because critical efficacy data and optimal application methods are lacking (Isman 1997, Ujváry 2002).

Although applications of essential oils as commercial crop protectants have been limited to date, many of these essential oils are safe to humans and currently are used widely in the food and cosmetic industries. Further, with their low mammalian toxicity and low potential for other negative environmental effects (Isman 2000, Ebbon 2002), the use of essential oils as crop protectants is highly appealing. The effects of individual oils or compounds on specific target pests can differ depending on their particular mode of action (Isman, 2000). Therefore, further studies are needed to establish and optimize their use in the field and enhance their potential for commercial development for specific pests.

Another naturally derived material used as a crop protectant is kaolin. Kaolin is an aluminosilicate mineral that can be sprayed onto plants, leaving a white washable residue. Kaolin-based particle films originally were developed for horticultural applications to reduce heat stress, sunburn damage in fruit trees (Glenn et al. 2001, Glenn et al. 2002). Additional research has focused on kaolin as an environmentally benign method to suppress arthropod pests and plant diseases (Glenn et al. 1999). It has been found to reduce the abundance of sucking and chewing pests of fruit crops (Knight et al. 2000, Puterka et al. 2000, Unruh et al. 2000, Knight et al. 2001), boll weevil on cotton (Showler 2002), whitefly on melons (Liang and Liu 2002), thrips on blueberries (Spiers et al. 2004), and other pests. As with essential oils, kaolin may affect these insect herbivores in several ways. Kaolin may alter the attractiveness or host location cues of plants; it forms a physical barrier that may interfering with insect movement and feeding (see Jifon and Syvertsen 2003, Puterka et al. 2005). Mortality of certain herbivores on kaolin treated plants has been observed, but the mechanisms have not been determined. Kaolin also can increase UV reflectance (Glenn et al. 1999), which has been shown to repel Frankliniella thrips (Kirk 1984, Terry 1997, Reitz et al. 2003, Momol et al. 2004).

Our preliminary data (Reitz et al. 2006) lead us to hypothesize that kaolin and the essential oils geraniol, lemongrass oil, and tea tree oil deter thrips and can be as effective as standard insecticides in managing thrips and tomato spotted wilt in tomato crops. As described above, certain essential oils can suppress other tomato pathogens (Wilson et al. 1997, Kalemba and Kunicka 2003), and kaolin can reduce foliar pathogens in certain crops (Glenn et al. 1999, Thomas et al. 2004). Therefore, plant essential oils and kaolin used for thrips and tomato spotted wilt management may have additional benefits in tomato production. Understanding the effects of novel management tools on plant health, vigor and yield are particularly valuable when planning cost effective programs for high input crops such as tomato.

The new strategies developed in this project will provide growers with more sustainable and environmentally benign tomato production practices. Users of this research will be farmers, IPM providers, extension personnel, and biorational crop protectant manufacturers. This mission-oriented project supports SARE’s long range goal of improving the sustainability of US agriculture by reducing the use of high-risk synthetic pesticide use and promoting the use of environmentally appropriate reduced-risk materials.

Project Relevance to Sustainable Agriculture

The proposed project aims to develop effective and practical tools for management of thrips and tomato spotted wilt on tomatoes, an economically important crop in the southern US. Thrips and tomato spotted wilt continue to be critical concerns for conventional growers in the southern US, and in a previous SARE funded survey project, insect and disease management were identified as the two most important issues facing organic farmers in the southern US (Boyhan 2003, LS02-142). Our project will address these concerns and contribute to more sustainable vegetable production through several mechanisms.

Management of thrips and tomato spotted wilt on tomatoes has relayed high input control measures, including extensive use of conventional synthetic pesticides. Not only do these chemicals, such as methamidophos, not provide satisfactory disease control, they also pose threats to consumers, growers, and the environment. In comparison, use of the reduced risk tactics proposed in this project will reduce these threats and will enhance the quality of life for farmers and consumers. Furthermore, the registrations for many synthetic pesticides, including methamidophos, are being cancelled because of the Food Quality and Protection Act of 1996, making the development of effective, environmentally benign crop protectants essential to meet the future needs of US agriculture. In line with that need, the plant essential oils that we propose to study are generally regarded as safe by the US FDA and EPA (GRAS under CFR 21 section 182.20), and kaolin is approved for organic farming use. Therefore, their use in place of synthetic pesticides could provide effective control of thrips and tomato spotted wilt, while improving farm worker and consumer safety, and environmental quality.

We will take a unique approach of using plant essential oils and kaolin synergistically to reduce the incidence of thrips and tomato spotted wilt. There has been increasing interest in the use of plant essential oils and other plant-derived materials as crop protectants. Although we are not aware of other SARE projects using plant essential oils to control insect vectors of plant diseases, our project would complement other SARE projects involving essential oils. One of the PI’s (M. T. Momol) is developing essential oils for control of bacterial spot and bacterial wilt on tomatoes (LS06-192). Researchers in Tennessee have examined plant-derived materials for controlling other tomato pathogens such as Phytophora and Pythium (LS03-147). A northeast SARE funded a project examined the use of plant essential oils for control of varroa mite in honeybee colonies and to improve overall hive health (LNE98-105). SARE has funded at least 17 projects involving the use of kaolin; however, none of these has examined kaolin as a method to control the vectoring of pathogens by insects. As this research evolves, there will be increasing demand for and uses for plant essential oils and kaolin in pest management. Expanding the uses of plant essential oils would complement other goals of SARE funded research in developing new markets for plant essential oil producers (see FNE01-358).

Our proposed research to look at mechanisms will help to optimize methods for using plant essential oils and kaolin in pest management programs. Determining the mechanisms by which essential oils and particle films affect insect biology and behavior is critical to deploying these materials efficiently in pest management programs for tomatoes and other crops.

Furthermore, these materials could produce multiple benefits to growers in terms of pest and disease management and crop production. Our preliminary studies show that these products deter thrips and reduce tomato spotted wilt. In addition, as described above, certain essential oils can suppress other tomato pathogens and kaolin can reduce foliar pathogens and physical damage in certain crops. Therefore, their use to suppress thrips and tomato spotted wilt would not be disruptive to overall tomato production. Rather they would enhance overall pest management in a sustainable and environmentally sound manner.

The proposed research will have immediate and longer term benefits to farmers, IPM providers and extension personnel. Our project will lead to improved use of kaolin in tomatoes and provide data to further stimulate interest in commercialization of plant essential oils as crop protectants. Therefore, this project supports the long-range goal of improving the sustainability of US agriculture by reducing use of high-risk chemicals through environmentally reduced-risk tactics.