Sustainable management of cereal leaf beetle
By S. Kher L. Dosdall H. Cárcamo
The cereal leaf beetle is an invasive Eurasian pest of cereals including wheat, oats and barley recently discovered in western Canada. It has been established in North America at least since 1962, and since then it has expanded its geographical range significantly. Management approaches include quarantine, insecticides, cultural control, plant resistance, and biological control. However, biological control with natural enemies has been the most successful strategy for control in North America following the successful establishment of its principal parasitoid, Tetrastichus julis introduced from Europe.
The cereal leaf beetle, Oulema melanopus L. (Coleoptera: Chrysomelidae), is a relatively recent alien invasive pest in western Canada that infests a range of agriculturally important cereal crops including wheat, oat and barley. Native to Eurasia the beetle was discovered in North America in 1962 in Michigan, U.S.A. The beetle has expanded its range in recent years to encompass most regions of cereal production in the U.S.A., and portions of Alberta, Saskatchewan and Manitoba.
In southern Alberta, beetle activity in the field begins from mid-April to May with the emergence of overwintered adults. Adults disperse to winter wheat fields, mate and begin ovipositing. Peak oviposition occurs in May. Larvae are active from May until July. Larvae are less mobile and do not usually move from one plant to another. Pupation occurs beneath the soil in July and new generation adults emerge in about three weeks. The adults feed for a short time on crop plants in late summer before dispersing to overwintering sites. Greater infestation levels have been observed in winter wheat than in spring cereals including spring wheat, oat and barley.
Host range and damage potential
The cereal leaf beetle attacks many wild and domesticated grasses. Although considered polyphagous within the family, the insect prefers: wheat, oat, barley > rye, timothy > fescue > grain sorghum, corn. Larval feeding leads to significant losses in crop yield quantity and quality due to reduced photosynthetic activity.
Most crop damage is caused by the late larval instars with the fourth instar alone responsible for about 70 percent of all damage. Feeding at the flag leaf stage is most damaging to crop yield. Adult feeding is characterized by uniform longitudinal incisions. Wheat seedlings are most prone to the attack by both larvae and adults. In Canada, yield losses may reach 30 percent.
Control initially relied heavily on the use of insecticides which included compounds like carbofuran (soil application) and endosulfan (foliar sprays). Synthetic pyrethroids such as permethrin, cypermethrin and fenvalerate were effective at low doses and were biodegraded by the plant, but were found to be lethal to the natural enemies of the pest, mainly the parasitoid wasp, Tetrastichus julis (Walker) (Hymenoptera: Eulophidae). Use of chemicals may adversely affect the survival and development of natural enemies thus hampering the process of natural control of the pest which might favour pest outbreaks. Seed treatment in barley with imidacloprid caused about 40 percent mortality, while foliar sprays caused about 90 percent mortality in the cereal leaf beetle population.
Host plant resistance
Trichomes or pubescence (plant hairs) deter feeding, oviposition or both, resulting in “non-preference”. Mechanisms of resistance in wheat have been widely explored and wheat demonstrates strong resistance mechanisms compared to oat and barley. Leaf pubescence in wheat can deter oviposition and affects hatchability, larval survival and adult feeding on resistant wheat varieties. The source of resistant germplasm for the cereal leaf beetle is concentrated mainly in Asia Minor and south-eastern Europe and initial efforts to control the pest focused on exploration of resistant germplasm in small grain host crops, such as wheat, and barley. Trichomes of pubescent wheat varieties contain silica which imparts indigestibility. Narrow-leaved cereal varieties also resist larval feeding by limiting the space for feeding and larval activity. Production of volatile compounds by some host species also has antibiotic effects on the larva. For example, a secondary volatile chemical, 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (also called DIMBOA), in corn negatively affects the growth of overwintered adults. Some important constraints in exploiting resistant germplasm for cereal leaf beetle control include difficulties in locating resistant sources, low success with crossing, narrow adaptation and more importantly, the association of resistance with lower yields. Nonetheless, host plant resistance deserves consideration as a component of integrated pest management.
Biological control with introduced natural enemies has been the most successful strategy for managing the cereal leaf beetle. The fauna of natural enemies of the cereal leaf beetle includes insect predators, parasitoids, mites and some bird species. The species of larval parasitoids introduced for biological control include T. julis, Diaparsis carinifer (Thomson) and Lemophagus curtus (Townes) (Hymenoptera: Ichneumonidae), and an egg parasitoid, Anaphes flavipes (Foerster) (Hymenoptera: Mymaridae).
Mass multiplication of T. julis in the laboratory for field release is difficult due to limited success in parasitoid reproduction and development. This gave rise to the successful concept of “field nurseries” in which the parasitoids are reared in a protected field area with deliberate cereal leaf beetle infestations to support parasitoid growth under natural conditions; parasitized larvae are relocated to infested fields. T. julis remains the most successful parasitoid of the cereal leaf beetle and the wasp is well established in North America due to its high synchronization with the host and capacity to track its host as the host range expands geographically.
Tetrastichus julis is a host-specific, bivoltine, gregarious larval endoparasitoid of the cereal leaf beetle. The parasitoid lays about four to six eggs per host larva and attacks all instars, but the young larvae are preferred. It overwinters in the larval stage within the pupal cell of its host at a soil depth of 5 cm. Overwintered females parasitize mostly early developing beetle larvae at the beginning of the season. The parasitized host larva dies after pupation. Parasitization is high in spring with a peak in mid-May to June. However, the second generation of adults also parasitizes late-maturing host larvae. Tetrastichus julis is equally active in oat, wheat and barley.
High temperatures during mid-June can induce a period of quiescence (called diapause) until favourable temperatures return74. In western Canada, especially in Alberta, the parasitoid is established and controls cereal leaf beetle populations to varying degrees depending on site and year. Parasitization occurs from mid-late May and continues until the end of June. Peak parasitization in June has been the usual trend. Second generation parasitoids parasitize late maturing O. melanopus larvae and such parasitoid larvae overwinter inside infested larval cocoons and start the cycle again.
The second major parasitoid of the cereal leaf beetle is the egg parasitoid, A. flavipes, which was first discovered from cereal leaf beetle eggs in Pandino, Italy in 1964. It is a minute species measuring less than 1 mm, and was first released in the U.S.A. in 1966. The parasitoid lays varying numbers of eggs in host eggs (with a maximum of eight eggs observed per host egg) which develop in about 10 to 11 days at 21 degrees C. However, the parasitoid is not as successful as T. julis in North America due to its asynchrony with peak oviposition activity of the beetle.
Research prospects and scope for integrated pest management
The current overview of the cereal leaf beetle as a pest in North America underlines the fact that using only a single control strategy cannot guarantee population control, and an integrated pest management system with judicious integration of possible approaches can optimize pest management. In the western Canadian context, the recent invasion by the cereal leaf beetle provides a unique and historically rare opportunity to study and understand the initial dispersal characteristics of the cereal leaf beetle. This is fundamentally important for understanding community assembly dynamics, and initial colonization and dispersal of a pest is the best stage during which to implement management efforts. In the Canadian context, there is opportunity to explore host plant resistance and the suitability of T. julis for the temperate continental climatic conditions that characterize principal areas of cereal production. Natural enemies should prove to have a significant role in controlling pest populations and their augmentation in the field can help ensure timely control of the pest. However, this requires a thorough understanding of the natural enemy complex associated with the pest in a given ecological niche and interrelations with the pest and its host plants. There is a dearth of studies in North America that attempt to explore such interactions and there is an opportunity to study the natural enemy complex of the cereal leaf beetle in its new habitat. Earlier studies have shown that climate change is likely to alter cereal leaf beetle distribution and abundance. However, it remains to be understood how its natural enemies, particularly T. julis, would cope with such environmental changes. It is important to understand parasitoid synchronization with beetle dispersal in new areas and in unique landscapes to better predict whether it will be successful in keeping pest populations in check. Finally, effective communication with the farming community on research achievements is crucial for ensuring that appropriate management strategies are implemented. The complete, unedited version of this Journal article can be found at www.prairiesoilsandcrops.ca. Prairie Soils & Crops: Scientific Perspectives for Innovative Management is a “peer reviewed” e-Journal that provides agronomists, producers, agrologists and certified crop advisors with current perspectives on various issues pertaining to soil and crop management on the Prairies.