Sunn pest of wheat and barley in the Islamic Republic of Iran: chemical and cultural methods of control

M. Javahery

IMPORTANCE OF SUNN PEST

Among pentatomid bugs, some species from the families Pentatomidae and Scutelleridae are serious pests of wheat and barley. These heteropteran insects, collectively known as sunn pests, have a wide distribution in palearctic regions, especially in southwest Asia, eastern Europe and North Africa where several Aelia and Eurygaster spp. are major pests of wheat. The biology, ecology, morphology and, to some extent, the physiology, of these insects are known. Despite considerable research conducted over the past 50 years on sunn pest control, however, most methods are not effective during outbreak seasons.

CEREAL CROP YIELD REDUCTION

Growing wheat and barley has been a common agricultural tradition over a period of several thousand years in the regions mentioned above. Wheat and, to a lesser extent, barley make up the basic diet of people living in these regions. In addition, barley is widely fed to livestock.

Wheat and barley are both grown under irrigated and rain-fed (dry-farming) methods. Average grain yields range from about 2.5 tonnes per hectare in irrigated fields down to about 1 tonne per hectare in rain-fed areas.

Among the Pentatomidae, several species from the genera Aelia, Carpocoris, Dolycoris and especially Eurygaster cause important grain yield losses, and are widely recognized as the major pests of wheat and barley. The bugs attack the stems of young, succulent wheat and barley plants, causing them to wither and die prior to head formation. They also feed vigorously on developing and mature grain. While feeding on stems or grain, they secrete proteolytic enzymes that affect the grain gluten. Consequently, the resulting flour loses its baking quality, even when only a small percentage of the grain milled into flour has been attacked. Unless the sunn pest is controlled, crop losses of up to 100 percent can occur during outbreak years.

BIOECOLOGY OF CEREAL PENTATOMOIDEA FROM THE GENERA AELIA, CARPOCORIS, DOLYCORIS AND EURYGASTER

Comparison of the major genera

A study of the comparative biology, ecology and reproductive biology of 15 species from the above genera that appear on wild and cultivated cereals has been made in Canada, the United Kingdom and Iran. Short research visits have been made to these countries to investigate the sunn pest under various ecological conditions.

The genus Eurygaster is placed within the family Scutelleridae, while the other three genera are in the family Pentatomidae. Among the Aelia spp., A. acuminata (L.), A. furcula F, A. rostrata Boh. And A. virgata Klug are the most common pentatomids on cultivated cereals and on grasslands. A. furcula and A. rostrata are major pests of wheat in Iran, Turkey and the southwest regions of the former Union of Soviet Socialist Republics. A. acuminata has a broad distribution from southwest Asia to western Europe and northwest Africa. It is also an important wheat pest in Morocco, but populations are low on cultivated wheat in other regions.

About 15 species of the genus Eurygaster are known on wheat in palearctic regions of Europe and Asia. However, five species, E. integriceps Puton, E. maura L., E. hottentata Fieber, E. austriaca Schrk. and E. testudinaria Gebb are serious agricultural pests.

Carpocoris spp. are rarely found on wheat in conjunction with Aelia and Eurygaster spp. They are a secondary pest of cereals in some areas of southwest Asia.

Dolycoris spp. are cereal pests in dryland wheat fields near forests. Populations occasionally increase to outbreak levels.

All species of cereal bugs are commonly found on wild graminaceous plants such as Agrostis, Avena, Bromos, Dactylis, Festuca, Lolium and Poa spp. In the absence of these wild and original food plants, sunn pest populations feed and develop on cultivated cereals, especially wheat. Eurygaster integriceps is perhaps the best example of this genus and is the most serious wheat pest in southwest Asia. In dryland wheat fields of western Iran, overwintered adults and nymphs of E. integriceps feed and develop on wild plants of graminaceous species such as Heteranthelium piliferum (Banks & Seland.) Hochst. which grow wild near wheat fields. In fact, all sunn pest species prefer feeding on the wild graminaceous plants mentioned above which frequently grow near cultivated wheat and barley fields. This has been commonly observed throughout the range of sunn pest in palearctic and nearctic regions.

Population dynamics of Aelia and Eurygaster

A long-term study of the population dynamics of Aelia and Eurygaster species in outbreak areas, including Waramin and Isfahan in Iran, indicated that changes in population densities of these insects and outbreaks are largely determined by external abiotic and biotic factors. Climatic conditions, especially temperature and rainfall, play an important role in sunn pest population dynamics. In mountainous wheat fields in western Iran, populations of these insects have been maintained on both cultivated wheat and on wild graminaceous plants. Among the natural enemies observed, scelionid egg parasitoids greatly reduced populations of A. furcula and E. integriceps in certain regions. The degree of parasitism in several areas, such as in untreated wheat fields in Chemastan in northern Iran, reached 96 percent in 1991. Long-term applications of chemicals in Iran are likely to have promoted sunn pest outbreaks. Figure 6 shows seasonal changes in the number of overwintered sunn pest adults and nymphs in wheat near Waramin and Isfahan from 1960 to 1992. Outbreaks occurred every six to eight years. These two areas have different ecological conditions; Isfahan has a very dry climate with only 100 mm annual rainfall, while Waramin receives about 254 mm of rain annually. Wheat and barley are cultivated under irrigation in both areas.

Figure 6 - Seasonal changes in the number of overwintered adults and larval instars of Eurygaster integriceps

Factors favouring mass multiplication of heteropteran stink bugs have been studied in detail in species from different climatic regions in Canada, the United Kingdom and Iran. High temperature was important to Aelia and Eurygaster spp. in subtropical regions, but adversely affected temperate species. Food resources affected fecundity in all the pentatomid species that live on cereals and legumes. Fecundity and longevity of sunn pest species were much higher on grains. In Aelia and Eurygaster, fecundity and longevity were greater on wheat and barley grain with water than on green shoots of these cereals (Figures 7 and 8). Fecundity of these pests was slightly higher on grain and green shoots of wheat than on barley. The mean female fecundity of E. integriceps on wheat grain and water was about 190 to 200 eggs, and on barley grain and water about 180 to 185 eggs at 28°C and 70 percent relative humidity (Figures 7 and 8). Fecundity, however, decreased by half when insects were kept on green shoots of wheat or barley at the same temperature or at low temperatures of 20 to 23°C. E. integriceps fecundity sharply decreased when it was bred under crowded conditions on both wheat and barley grain with water. This is shown in Table 13.

Figure 7 - Fecundity of E. integriceps in relation to food and temperature

ABIOTIC FACTORS

Among climatic factors, temperature influences most activities in pentatomids, as it does in all insects. Sunn pest is able to adapt to temperature changes ranging from -30°C in winter to 45°C in summer in dry regions.

Continuous rainfall retards sunn pest activities. Long periods of humidity over 90 percent, under shelters in grasses in temperate climates, on host plants in the mountains or at overwintering sites in forests and woodlands, cause mortality in all sunn pest species. Rain and snow-melt may cause high mortality, especially if a cold spell freezes the ground during winter hibernation. Sunn pest adults and nymphs, except for first instars, are active on sunny and calm days and inactive in the breeding habitat on windy and cool days. Newly developed adults have a negative phototactic response during summer aestivation and during autumn and winter hibernation.

Figure 8 - Fecundity of E. integriceps in relation to food and temperature

The effect of climatic changes on the populations of the dominant species Aelia furcula and Eurygaster integriceps was noticed in 1972 and 1992. Population density of A. furcula, the second most important sunn pest species after E. integriceps, increased three times as quickly as E. integriceps during three consecutive rainy seasons from 1970 to 1972. A. furcula sex ratios changed during this period to about three females to one male.

TABLE 13 - Comparative sunn pest investations on barley

Crop	Number of sunn pest pairs
	1	5	10
Barley	180	-	115
Wheat	-	-	190

DIAPAUSE AND NUMBER OF GENERATIONS

Sunn pests, like most bugs, diapause as adults. Adult diapause in Aelia, Carpocoris, Dolycoris and Eurygaster is influenced by both photoperiod and temperature. Generally, sunn pest populations are univoltine, with the exception of one or two Aelia species, and have an obligate diapause throughout their geographical range. The duration of adult diapause, however, lasts from four to six months and begins in the hot, dry summer and ends in winter. The diapause is characterized by arrested development of the reproductive organs. As with other insects, characteristics of sunn pest diapause include:

· strongly reduced basal metabolism;
· low water content and high fat content;
· cessation of morphogenesis.

MIGRATION

Sunn pest dispersal occurs as true migration (E. integriceps) or as short flights (A. acuminata). It is generally accepted that newly emerged Aelia, Carpocoris, Dolycoris and Eurygaster migrate with prevailing winds from oviposition sites to aestivation sites on mountains or in nearby woodlands and forests surrounding breeding habitats.

In both types of dispersal, males and females are sexually immature before, during and after moving to aestivation sites. Migration, either over a long distance (25 km or more in the case of E. integriceps) or over a shorter distance (less than 25 km in the case of Aelia, Carpocoris and Dolycoris), results in aestivation in a cooler climate. In both temperate and subtropical areas, these insects undergo an obligate diapause for two to four months in late summer and early autumn under bushes at high altitudes or on trees and bushes in forests and woodlands. In early autumn, after a cool period and rain, they descend to lower altitudes, frequently on the southern slopes of hills or mountains. Migration distance is generally shorter in subtropical and temperate climates as exemplified by Eurygaster in California, the United States. A. acuminata may undergo both long- and short-distance dispersal.

All sunn pests hibernate until temperatures rise in spring when they move to cereal fields to feed and mate. Overwintered bugs then die.

In almost all cases, aestivation and hibernation periods are in an adaptive phase of obligatory reproductive diapause until the beginning of favourable conditions in spring. In both sexes and with both migrated and stationary individuals, feeding in spring is essential prior to the first mating and oviposition.

Sunn pests overwinter as adults and migratory flight results from an endocrine deficiency syndrome concomitant with early adult life and ovarial immaturity, ovarial diapause and structural polymorphism.

CURRENT CONTROL METHODS

Chemical control

The use of pesticides against sunn pests began during the Second World War with the application of dinitro-orthocreasol in Iran. This compound was sprayed on adult E. integriceps and Aelia which aggregated on or under host plants of the genera Artemisia, Astragalus, Acantholimon and Acanthophyllum during aestivation and hibernation in the Ghara-Aghatch mountains east of Teheran.

In 1952, organochlorine (DDT) was used against Aelia and Eurygaster and gave good control. DDT was the most effective and widely used chemical for controlling sunn pest until 1960 when DDT resistance was observed in E. integriceps nymphs. In 1959, Dipterex, an organophosphate, was found to be highly effective on adults and nymphs and was recommended for sunn pest control. Both DDT and Dipterex were widely used until 1970. Two other ester phosphoric compounds, Lebaycid (= Baytex = fenthion) and Sumithion (= Folithion = fenitrothion), were tested and have been used against sunn pest since 1965 and 1973, respectively.

Applications of a 50 percent emulsion of Sumithion 50% against sunn pest in Iran have increased from 50 000 ha in 1968 to nearly 1 million ha in 1988 and 1991 (Figure 9). Estimates of the return value of the crop after spraying for sunn pest for 1991 are given in Table 14.

DISADVANTAGES OF CURRENT CONTROL MEASURES

Large quantities of organophosphate insecticides are now used each year to control sunn pest in Iran. Use was especially high during outbreak years from 1987 to 1992. From 1970 to 1980, about 4.5 million ha of wheat and 1.5 million ha of barley were grown each year in Iran. In 1992, an estimated 6 million ha of wheat and 2 million ha of barley were grown. There has been an increase of 2 million ha in the area of both wheat and barley under cultivation in Iran over the period 1968 to 1992.

From 1968 to 1978, approximately 100 000 litres of Sumithion 50 ec were used per year. Use of this insecticide against sunn pest has increased since 1978 to nearly 1.5 million litres in 1987 to 1992 (Figure 9).

Each chemical insecticide used against sunn pest in Iran has been effective during the initial ten years or so of its use. E. integriceps became resistant to DDT in 1955, to Lebaycid in 1975 and to Sumithion in 1985. The effective life of chemical insecticides against sunn pest appears to be about ten to 15 years, after which pesticide resistance seriously reduces their effectiveness.

It was thought for many years that using new organophosphate compounds would end sunn pest outbreaks. However, sunn pest outbreaks have occurred three times in Iran during the period 1963 to 1993 (Figure 6).

SUGGESTED MANAGEMENT SYSTEM

Cultural methods

Resistant varieties. The present method of wheat and barley cultivation in small, widely dispersed fields contributes to sunn pest outbreaks in parts of Iran. Some small fields and those under high-power electricity cables are rarely sprayed. In addition, small wheat and barley fields in forests and in orchards have provided additional habitats for sunn pest and are not sprayed. Crops other than wheat and barley should be substituted in these areas.

Figure 9 - Chemicals used and area sprayed against sunn pest in Iran

TABLE 14 - Economics of sunn pest control using chemical insecticides in Iran (1991)

Total area sprayed	811195 ha
Average grain yield	1 tonne/ha
Percentage of grain damage in untreated fields	50%
Crops protected	405597 tonnes
Estimated value of protected crops	US$ 67599500
Spray costs/ha	$ 33
Total spraying costs	$ 26769435
Price of wheat/tonne	$ 165
Return value of crop	$ 40830065

Host plant resistance has received substantial consideration during the past 30 years and is considered a mainstay of integrated pest management (IPM) programmes worldwide. Both Aelia and Eurygaster appear to prefer wild graminaceous plants and starve on diets lacking the proper feeding stimuli. The level of plant resistance against sunn pest has not been well studied on either local or exotic wheat varieties.

Most wheat grown in Iran has been of the varieties Bezostaya, Omid, Roshan, Sardari and Tabassi. In 1976, a short-stem Mexican wheat was imported and grown in the main sunn pest outbreak areas. E. integriceps has shown a slight preference for native wheat over the Mexican variety when grown side by side in the Saveh and Waramin experimental fields near Teheran.

From 1987 to 1990, an experiment to test for resistance in wheat and barley varieties was carried out by R. Talai at the Department of Plant Protection of the University of Teheran. During sunn pest outbreaks from 1987 to 1990, she tested 12 varieties of wheat and barley on 1 000 m² at Karaj. Damage caused by overwintered and new-generation adults, as well as by nymphs (second to fifth instars), to leaves, stems, heads and seeds, and effect on yield and baking quality of cultivated wheat and barley were studied in relation to sunn pest population density. Results indicated resistance in several varieties (Table 15).

Developing and culturing early-ripening varieties of wheat and barley. Newly emerged Aelia and Eurygaster feed for about two weeks before migrating to aestivation areas. Feeding at this time is essential for migration and survival. Early-ripening varieties of wheat and barley should be grown in areas at risk of sunn pest outbreaks.

Two-stage harvesting. Two-stage harvesting involves harvesting the crop about two weeks before the heads are fully ripened. At this growth stage, sunn pest are not yet adults. The harvested crop is left in the field in windrows for about two weeks where it partially dries before being collected and threshed. This keeps the sunn pest from feeding sufficiently and delays maturation. The insect is unable to store adequate fat reserves and to complete migration successfully. Two-stage harvesting is most applicable to large-scale mechanized cultivation and is discussed at length by Jorgenson (1986).

TABLE 15 - Resistance of wheat and barley varieties to Eurygaster and Aelia in Iran

Variety	Resistance level
Wheat
Sardari	Medium
Bezostaya	Medium
Roshan	Medium
Tabassi	Medium
Omid*	Medium
Ghods	Susceptible
Karaj 1	Susceptible
Barley
Arivat	Medium
Valfajr	Medium

* Omid had very good baking quality.

SELECTIVE INSECTICIDE USE DURING OUTBREAKS

During the last 20 years, Sumithion has been the only emergency tool used in different parts of Iran when sunn pest populations exceeded economic thresholds. When misused, this compound is toxic to sunn pest hymenopterous egg parasitoids, particularly those attacking E. integriceps.

Scelionid wasps which attack sunn pest eggs are important in maintaining sunn pest populations below the economic threshold. Selective insecticides should be identified and used in conjunction with proper timing to preserve field populations of natural enemies.

AUGMENTATION OF NATURAL ENEMIES

Although large quantities of organochlorine (DDT) and organophosphate (Dipterex, Lebaycid and Sumithion) pesticides have been sprayed in breeding areas of scelionid egg parasitoids over the past 40 years, egg parasitism by Trissolcus and Telenomus reached 75 to 78 percent in Waramin and Karaj in 1990 and 1992, respectively. In selected untreated wheat fields, such as near Tchemastan in northern Iran, parasitism reached 95 percent in 1991. These wasps have potential for use in a sunn pest integrated management programme.

There have been several long-term studies on the impact of scelionid egg parasitoids on sunn pest in Iran, as well as on a number of other pentatomids on wild graminaceous plants in Canada and the United Kingdom. Scelionids appear about two weeks after the onset of oviposition by sunn pest. The egg parasitoids are more susceptible to cool temperatures in early spring than the sunn pest. Eggs laid by bugs early in the season are frequently unparasitized.

GROWER COOPERATION

The timing of planting and the cultivation of cereals in certain dryland areas could be restricted by growers in order to reduce sunn pest populations. Wheat and barley are grown mainly during October and November in the rain-fed and irrigated areas of Iran. However, some wheat is also grown in early spring in the rain-fed areas of western Iran. Two-thirds of the wheat and barley fields are located in western Iran where there is normally adequate annual rainfall. In other areas, cereals are grown under irrigation. When there is insufficient rainfall, grain yield is low and farmers do not harvest their fields. The unharvested fields provide additional food for sunn pest populations. The first migrant sunn pests arrive in cereal fields in late March, but a high percentage do not migrate until April. Sunn pests are found mostly in early-planted fields distributed among late-sown fields. These fields are not sprayed because the populations of sunn pest are below the economic threshold in March and April over much of the pest’s range. A noticeable percentage of the sunn pest population reproduces in these fields each year.

Growers should avoid the unnecessary use of pesticides on cereals and adjacent crops in order to maintain natural enemy populations, especially those of scelionid egg parasitoids.

FUTURE NEEDS AND CONSIDERATIONS

Bioecological studies of Aelia and Eurygaster began in southwest Asia nearly 200 years ago. Chemicals were used against sunn pest in aestivation and hibernation habitats over 60 years ago. It is discouraging to see that sunn pest is still a problem. The entire sunn pest ecosystem must be properly understood and current management tools should be correctly used within the context of IPM. IPM programmes should be based on a sound understanding of the ecology of sunn pest.

REFERENCES

Donskoff, M. 1985. An approach to the integrated protection of cereals against sunn pest in Iran. FAO Report, Rome, FAO.

Javahery, M. 1972. Annual technical report on sunn pest to the Plant Pests Research Institute. Teheran, Iran. 72 pp. (Unpublished)

Javahery, M. 1973a. Population dynamics of Aelia and Eurygaster spp. in some regions in Iran.

Javahery, M. 1973b. Integrated pest management of sunn pests in Iran. Teheran, Iran, CENTO Publication.

Javahery, M. 1978. Importance and methods of control of sunn pest in Iran. Bulletin of Plant Pests Protection Organization, Teheran, Iran.

Javahery, M. 1988. Migration in Pentatomoidea. Abstract, 18th International Congress of Entomology, Vancouver, Canada.

Javahery, M. 1992. Sunn pests survey and their control. Abstract, Sunn Pest Conference, University of Teheran, Iran.

Jorgenson, M. 1986. Two-stage mechanized harvesting of wheat and barley to control sunn pests in Iran. Technical Report to the Ministry of Agriculture of Iran. Humbolt, Saskatoon, Canada, Prairie Agric. Mech. Institute.

Talai, R. 1991. Research on resistant varieties of wheat and barley against sunn pests. University of Teheran, Iran. (M.Sc. thesis)