The Tomato Psyllid: A NEW PROBLEM on Fresh Market Tomatoes in California and Baja Mexico
The adult psyllid is a small insect (about 1/8th inch) that resembles a cicada. The adults have white or yellowish markings on the thorax, clear wings, and lines on the abdomen that separate segments.
The eggs are laid on stalks, and look a lot like green lacewing eggs. However, the eggs of the psyllid turn a pink color a few hours after oviposition. The eggs can be laid anywhere on the leaves, but are most common on the undersides and along the leaf margins. Eggs on the leaf margins are easiest to see. A hand lens is useful for observation.
The nymphs feed most often on the undersides of the leaves. The larger nymphs have wing buds, which makes them easy to distinguish from whitefly nymphs. In addition, tomato psyllids do not cover themselves with a wax, as is seen with whiteflies. Finally, tomato psyllid nymphs can move if disturbed.
In the past two years, additional major outbreaks of the psyllid have occurred. In Ontario, Canada, greenhouse tomato growers experienced losses. Potato growers in Washington State suffered through a very substantial outbreak that was so novel that initially the specialists could not identify the problem. Significant outbreaks have also occurred in Mexico near the Rio Grande and in Colorado on both potatoes and tomatoes. Fresh market tomato growers in Baja, Mexico have suffered major economic losses from the psyllid. During 2001, over 85% of the mature tomato plants were destroyed; total fruit losses were even higher. Thus, the problem appears to be increasing in both geographic scope and economic importance.
The tomato psyllid problem is increased by several factors. First, the nymphs (and perhaps adults, Daniels 1954) inject a toxin while feeding on the leaf that causes death in transplants, stunting, chlorosis and curling of leaves in preflowering plants, and either no fruit production or overproduction of very small, non-commercial grade fruit in larger plants (Pletch 1947, Al-Jabar 1999). These symptoms are collectively known as "psyllid yellows". As few as 30 nymphs per plant reportedly can cause these symptoms on established tomato plants (Blood et al. 1933), but the varieties we use today are much different and more research is needed. Carter (1950), working with varieties popular in the late 1940s, determined that symptoms occurred on transplants from the feeding of a single nymph. No data are available on thresholds causing psyllid yellows on current commercial tomato varieties, but Abernathy (1991) found >40% yield loss in 8 varieties of greenhouse tomatoes grown in Colorado. The rapid developmental times coupled with maximum oviposition in excess of 1400 eggs/female allows populations to build explosively. These pests also have an extensive range of acceptable hosts, including species in 20 plant families, but solanaceous species (tomatoes, potatoes, nightshade) are preferred (Wallis 1955). The lack of information on reliable monitoring techniques makes early detection of these insects very difficult, leading growers in many areas to apply prophylactic pesticide treatments. There is some indication from recent presentations at meeting in Mexico that the psyllids may also transmit a pathogen, but no reports have been published that verify this concern.
Movement in California
The tomato psyllid was originally described from collections made in Colorado and New Mexico in the late 1800s (Pletch 1947). Although early reports stated that this pest was only found in Utah, Colorado, and parts of Wyoming (Richards 1927), sporadic early populations have been reported in California (Pletch 1947). Following major outbreaks in Colorado, Wyoming, and Montana from 1910-1945, the psyllid almost never reached economic levels. As a result, the insect does not appear in the California Tomato IPM Manual, and has only a passing mention in the CA Potato IPM Manual (even the pictures in the IPM manual are from a Colorado colony).
Relatively little information is available on the movement patterns of this pest in the USA. Romney (1939) indicated that the tomato psyllid was developing large spring migratory populations on the misnamed "overwintering breeding sites" in Southern Arizona from January through May on wild Lycium spp. Pletch (1947) later pointed out that the annual originating populations occurred much further south, and migration from Southern Texas (near the Rio Grande River) and even further south in Mexico provided the individuals that inoculated the 'overwintering breeding sites' each year in not only Arizona, but New Mexico and a small desert area at the extreme eastern edge of California near Needles in the Palo Verde Valley. Thus, given the recent outbreaks just south of San Diego, we are unsure if populations in California originate in Baja, Mexico or from much further south-east near Laredo, Texas. We also do not know how far this pest will spread in California. We found large populations in old plantings of fresh market tomatoes in Orange County this year, but did not have an opportunity to see how far north the pest moved.
MONITORING
Remarkably little is known about monitoring, economic injury levels (numbers of insects per plant that result in crop loss), alternative pest control strategies, or plant resistance to the psyllid or the toxin. Al-Jabar (1999), studied monitoring and control of tomato psyllid in greenhouses and showed that yellow, orange or green sticky traps will catch tomato psyllids. He also determined that a trap placement near the tops of the plants is optimal. So, at the current time, yellow sticky cards can be used as an early indicator of psyllid movement into an area, and plants on the field margins can be visually checked for eggs and nymphs.
Naturally occurring biological control of tomato psyllid has not been studied under California conditions, but the potential may not be great as beneficials have not proven effective elsewhere for suppression. There are no available field studies with predator augmentation, but some predators apparently will feed on the tomato psyllid if confined with the psyllid in small cages (Knowleton 1934, and references therein). In Colorado, minute pirate bugs and big-eyed bugs may become common, but do not suppress tomato psyllid outbreaks in the field (Cranshaw 2002). In the only report of biological control from California, Compere (1943) found a parasitic wasp, Metaphycus psyllidus Compere (Encyrtidae) attacking tomato psyllid nymphs, but there was no evidence presented that this parasite provided any significant control. Thus, the potential for biological control by predators and parasites in California is unknown. Other parasites that have been studied generally attack too late in the psyllid life cycle to stop the onset of psyllid yellows. At this time, biological control does not appear promising as a control strategy. However, more work needs to be conducted to make sure potential agents or strategies have not been overlooked.
CHEMICAL CONTROL
Relatively few reports are available on control using chemicals other than organophosphates or pyrethroids. We know that use of these materials in California causes a loss of biological control agents, resulting in outbreaks of secondary pests such as Liriomyza leafminers and spider mites (Trumble 1990, 1998). Most reports indicate that carbamates cause psyllid populations to increase, so these materials should be used with caution if psyllids are present.
In a series of over-the-top spray trials, some neonicotinoids and spinosad appeared promising in greenhouses. Because imidacloprid (a neonicotinoid) is often injected through the drip lines in California (a practice that fits our IPM program because it does not effect beneficial insects), additional tests will be needed to determine if this material will provide acceptable control in the field. Other tests with neem, and bacterial/fungal preparations did not provide as much initial suppression, but did have a positive effect under greenhouse conditions (Al-Jabar 1999). No information is available on insect growth regulators such as pyriproxyfen, harpin proteins that stimulate plant defenses, or the hydrophilic repellent particle films that have proven effective on pear psyllids (Glenn 1999).
We are submitting a grant to examine what compounds work against psyllids, how these pests move into and within California, and what critical threshold levels are for the major tomato varieties used in California. Should the grant be funded, I will inform the California Tomato Commission of the results in a timely fashion.
John Trumble
Thanks to the UC IPM Program for permission to use their photographs.
References
Abernathy, R. L. 1991. Investigations into the nature of the potato psyllid Paratrioza cockerelli (Sulc). M. S. Thesis, Colorado. State Univ., Fort Collins 54 pp.
Al-Jabar, A. 1999. Integrated pest management of tomato/potato psyllid. Paratrioza cockerelli (Sulc) (Homoptera: Psyllidae) with emphasis on its importance in greenhouse grown tomatoes. Ph.D. Dissretation, Colorado State University 52 pp.
Blood, H. L., B. L. Richards and F. B. Wann. 1933. Studies of psyllid yellows of tomato. Phytopathology 23: 930.
Carter, R. D. 1950. Toxicity of Paratrioza cockerelli to certain solanaceous plants. Ph.D. Dissertation, University of California. 128 pp.
Compere, H. 1943. A new species of Metaphycus parasite on psyllids. Pan Pacif. Entomol. 19: 71-73.
Cranshaw, W. S. 2002. Potato/tomato psyllid. High Plains IPM Guide: Potato XXI. (see also http://highplainsipm.org/HpIPMSearch/Docs/PotatoTomatoPsyllid-Potato.htm).
Cranshaw, W. S. and J. D. Donahue. 2002. Tomato psyllid. High Plains IPM Guide: Eggplant, pepper and tomato XXIV-6.
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Trumble, J. T., W. G. Carson and K. K. White. 1994. Economic analysis of a Bacillus thuringiensis-based IPM program in fresh market tomatoes. J. Econ. Entomol. 87: 1463-1469.
Wallis, R. L. 1955. Ecological studies on the potato psyllid as a pest of potatoes. USDA Tech. Bull. 1107: 25.
Zink, R. T. 1998. Colorado Agricultural Statistics Service, 1998 Colorado Pesticide Guide, Vegetable Crops