© 2004 Plant Management Network.
Trionymus haancheni McKenzie: A New Pest of Barley in Idaho
Juan M. Alvarez, Entomologist, Aberdeen Research and Extension Center, University of Idaho, Aberdeen 83210
Corresponding author: Juan M. Alvarez. firstname.lastname@example.org
Alvarez, J. M. Trionymus haancheni McKenzie: A new pest of barley in Idaho. 2004. Online. Plant Health Progress doi:10.1094/PHP-2004-0315-01-HM.
A new insect pest of barley, Trionymus haancheni McKenzie, sometimes referred to as the Haanchen barley mealybug, was discovered for the first time in Idaho near Soda Springs during June 2003 in a commercial barley field. Barley plants were in the mid-boot stage and most had fully developed flag leaves. Surveys since then have detected this pest in nine eastern Idaho counties: Bannock, Bingham, Bonneville, Caribou, Fremont, Jefferson, Madison, Power, and Teton (Fig. 1). It has been found aggressively feeding in great numbers on barley plants of different varieties (Tradition, Baronesse, and Harrington), mostly under dryland production, typically just above the soil surface. In addition to infestations at the soil surface, adults and nymphs also occur along the stems under the leaf sheaths of the plant.
Spring barley is an important crop in Idaho, with approximately 900,000 acres harvested annually (10). Approximate planting dates for major spring barley growing areas in the eastern crop reporting district where the insect was found first are mid-April to mid-May.
Samples of the insects sent to Drs. Greg Evans and Gregory Hodges, mealybug taxonomic specialists in the Florida Department of Plant Industry, were identified as Trionymus haancheni McKenzie (Homoptera: Pseudococcidae). This species was first described in California during the 1960s by Howard L. McKenzie (7), though it had been recognized there by farmers during the early 1950s as a pest of Haanchen barley in the Tulelake area of Siskiyou County (9).
This report is the first documented detection of T. haancheni in Idaho. Because the insect was previously unknown in Idaho and seemingly has not been economically important in any other states for at least 40 years, information about pest biology, damage potential, and control measures is quite limited. This publication summarizes what is known about T. haancheni from the published literature, as well as from field surveys in Idaho and laboratory observations at the Aberdeen Research and Extension Center of the University of Idaho during 2003.
The first signs of mealybug presence are masses of cottony appearance at the bases of the plants (Fig. 2). These cottony masses are the ovisacs (cottony clusters of eggs) of the mealybugs. Both nymphs and adults injure barley plants. They feed with sucking mouthparts and reduce the amount of chlorophyll in the leaves, causing extensive yellowing and browning of the foliage. Observational studies in the laboratory with field-collected adult females showed that 10 mealybugs per plant can cause leaf-yellowing symptoms within a week. Severe infestations in commercial fields eventually kill the plants (Fig. 3). Some species of mealybugs inject toxic saliva into the plant host (2,5). It is also possible that the injury symptoms in barley plants are caused by T. haancheni injecting toxic saliva into the plants.
In addition to direct feeding injury to barley plants, T. haancheni can damage the crop indirectly by producing a sticky sap-like substance called honeydew. Honeydew has the potential to reduce grain quality. Barley growers in California during the 1950s reported that honeydew accumulations in commercial fields were extensive enough to clog combines at harvest (7). Idaho growers in areas where mealybug infestations were confirmed during 2003 likewise reported that they had experienced the same problem with honeydew during the 2002 harvest. Growers, however, were not aware that these insects were present in their fields.
It seems likely that T. haancheni has been present in Idaho for several years but was either unnoticed or unrecognized until 2002 and 2003 when population densities reached damaging levels. The early-season damage symptoms to foliage observed during 2003 in Idaho had not been previously reported. Mealybug infestations in early July 2003 already were severe enough to produce noticeable amounts of honeydew in some commercial barley fields, suggesting that harvest problems would be worse during 2003 than in 2002.
Trionymus haancheni primarily damages barley. However, these mealybugs may cause differential injury among barley varieties. Differences in numbers of mealybugs were observed on different barley varieties in the field. Twenty plants of three different varieties from infested fields separated by less than one kilometer near Soda Springs, Idaho, were evaluated, and the numbers of adults on each plant were counted. The average numbers (± standard deviation) of adults per plant found on the varieties Tradition, Baronesse, and Harrington were 44.8 ± 20.66, 9.0 ± 3.16, and 3.4 ± 1.84, respectively. On the other hand, the variety most severely damaged by this insect in Ashton, Idaho (Fremont County), was Harrington. Trionymus haancheni also has been observed feeding on wheat and several grasses. McKenzie (8) reported that this insect was found under leaf sheaths of another grass, Elymus sp. (Gramineae), in California.
At this time, the extent of economic damage that infestations of T. haancheni will cause in Idaho is unknown. It also is too soon to predict the extent to which this pest ultimately might establish itself beyond its current nine-county distribution.
Description and Life Cycle
The name mealybug is derived from the waxy secretions that cover the bodies of these insects. The wax makes mealybugs look like they have been sprinkled with finely-ground, powdery, white meal. The adult female is quite small but is visible without magnification, reaching a length of approximately 5 mm. As noted, the body of some females is covered with a white, waxy secretion that extends as thin, wispy filaments along the edges of the body and at the posterior end of the body (Fig. 4). The body of the mealybug is elongate-oval, segmented, rather slender, and has well-developed slender legs. According to MacKenzie (7), the dorsal multilocular disk pores present on the last three or four abdominal segments differentiate T. haancheni from others species in the same genus. The number of cerarii (pore types used as distinctive characteristics in mealybug taxonomy) are reduced, with only the anal lobe and penultimate pairs present. The dorsal body setae are narrow and slender. Antennae have eight segments.
Only an expert using a good microscope can make a precise identification of T. haancheni. Although a second species of mealybug -- Trionymus utahensis (Cockerell) -- also was identified in Idaho samples, one can conclude with some confidence that the insects causing damage on barley in eastern Idaho and fitting the descriptions here were T. haancheni. Trionymus utahensis was originally reported as Pseudococcus neomexicanus var. utahensis (Cockerell) and was reported for the first time feeding on Elymus sp. in Salt Lake City, Utah (4). McKenzie (8) reported T. utahensis feeding on various grasses in the USA, and it was also reported in British Columbia, Canada, feeding on Elymus piperi Bowden and Agropyron sp, both plant hosts in the family Gramineae (6).
Adult males have not been seen on infested plants in Idaho and were not reported by Mackenzie in the original description of the species (7). Males of other mealybug species are the only winged life stage. All other life stages of this mealybug disperse short distances by crawling from plant to plant, or over longer distances on wind currents and as accidental hitch-hikers on infested plants or soil transported by wildlife or human commerce.
Females choose protected places to position the egg sacs. Eggs are deposited in loose, cottony wax. These cottony egg sacs usually are deposited on the lower part of the plant close to the roots. Egg sacs also were observed under the leaf sheaths of the plant (Fig. 5). Preliminary observations in the laboratory with field-collected nymphs placed individually in leaf cages (Fig. 6) revealed that a single female can lay as many as 256 eggs in a single ovisac during a week and 400 eggs during two weeks. This may indicate that reproduction occurs asexually in the absence of males. The eggs are pink-red and not visible without the aid of magnification.
Crawlers, the most mobile nymphal stage (Fig. 7), disperse to find suitable sites for feeding on plant sap. Crawlers also can be transported to other plants by wind, people, or animals. Crawlers develop through several successive instars that resemble small adults, each of which has legs and can move, until the adult stage is reached, after which the cycle repeats. The number of generations in Idaho still is unknown, but all instars can be found at a single time on a plant host.
Possible Conditions Influencing Mealybug Infestations
The circumstances that explain how this pest established itself in Idaho and what environmental or biological factors account for population increase are not known. Mild winter conditions in southeast Idaho during the past few years perhaps explain increased population densities. One could also speculate that outbreaks are related to the elimination of mealybug parasitoids after the application of insecticides directed against other barley pests such as cereal leaf beetle, cutworms, and aphids.
The most basic elements of an integrated pest management program are lacking for T. haancheni. Formal recommendations for field scouting do not exist, nor are there established economic thresholds. No insecticides currently are registered for use against this insect. Mealybug control tactics in other crops typically target the small, highly mobile crawler stage because it tends to be more vulnerable than the later, larger life stages. Insecticide applications often are timed for the week after egg-laying begins, to kill the nymphs before they develop to the egg-laying adult stage. This approach is necessary because some mealybug species consist entirely of females that reproduce asexually. Consequently, every individual is capable of increasing the infestation. Coupled with a short generation time, the ability to reproduce asexually can allow mealybug infestations to increase quickly to damaging levels.
The concealed feeding habits of T. haancheni, and the fact that the eggs are protected inside the cottony ovisacs would further complicate management attempts and limit insecticide use in barley because insects sheltered under leaf sheaths or ovisacs, would be protected from contact sprays. Seed treatments, foliar-applied contact insecticides that also have fumigant action (so that the chemical penetrates to insects behind leaf sheaths), or systemic insecticides might provide some control of mealybugs. However, no insecticides are registered specifically for this use, and none have been tested for cost-effectiveness under Idaho conditions of barley production.
Repeated insecticide applications likely would be necessary to reduce levels of infestation of mealybugs. However, experience with other mealybugs in different plant hosts has shown that insecticides by themselves are not effective (1). Applications of broad-spectrum insecticides are known to contribute to mealybug outbreaks in fruit trees, small fruit crops, and pine orchards by eliminating naturally occurring biocontrol agents that otherwise keep mealybug infestations at non-damaging levels (3). Biological control with parasitoids and predators has been the most effective and long-lasting management option with some other species of mealybugs.
I thank Rich Novy (USDA ARS, Aberdeen Idaho), Ed Bechinski (PSES, University of Idaho), and Ding Johnson (PSES, University of Idaho) for reviewing the manuscript. I also thank Monica Wiebe and Yadira Medina for technical support. Also, I gratefully acknowledge the editor and two astute referees for their helpful comments and suggestions to improve this paper. This is Idaho Agricultural Experiment Station manuscript #03744.
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