An Overview of the Lesser Cornstalk Borer
Management Program at HARC-HSPA, 1986 -2002
Hugh A. Smith and Asher K. Ota
The lesser cornstalk borer (Elasmopalpus lignosellus Zeller) (Lepidoptera: Pyralidae) was first discovered infesting Hawaiian sugarcane on Kauai in July 1986 (Chang and Ota 1987). By the following year, it had spread to all of the sugarcane growing islands (Chang and Ota 1988). The lesser cornstalk borer attacks many agronomic crops, including peanut, soybean, and cotton. Sugarcane is its principal economic host in Hawaii. The seed corn industry in Hawaii is in theory susceptible to losses from lesser cornstalk borer, but plots of corn grown for seed tend to be well-irrigated and intensively sprayed, limiting opportunities for lesser cornstalk borer growth. Lesser cornstalk borer also develops on several weeds that are common to sugarcane growing areas of Hawaii. Nutsedge (Cyperus rotundus L) is one of the most important wild hosts of lesser cornstalk borer in Hawaii.
The adult female of the lesser cornstalk borer lays eggs on or near the base of the stalk of emerging sugarcane. The larva then burrows into the stalk at or just beneath the soil surface, damaging the vascular system and allowing pathogens to enter the plant. Lesser cornstalk borer kills the central growing shoot, producing the characteristic "deadheart" symptom. Larvae may attack six or more shoots in succession before pupating. The lesser cornstalk borer can complete two generations during the 2-3 months during which a new field is susceptible.
In the late 1980s, Hawaiian Sugar Planters' Association (HSPA) entomologists Vincent Chang and Asher Ota evaluated chemical, varietal, cultural, and biological approaches to managing lesser cornstalk borer in sugarcane (Chang and Ota 1989, 1990). Chang screened a range of registered and experimental insecticides, including biological pesticides (Beauvaria bassiana and entomogenous nematodes). Pesticides were evaluated under dry and irrigated conditions, including chemigation. Neither conventional nor experimental pesticides provided a high level of control. Ten sugarcane cultivars were evaluated for lesser cornstalk borer resistance in 1987. H77-6694 and H77-6417 had statistically fewer deadhearts than other cultivars, although in the case of H77-6694, this was due to the generation of a high number of shoots. High tillering capacity is a desirable quality in sugarcane cultivars that will be ratooned where lesser cornstalk borer is a problem.
It was determined early on that lesser cornstalk borer thrives in dry soil, and that sugarcane is only susceptible to lesser cornstalk borer damage during the first few months of growth, before the first nodes are formed. Any agronomic practice that enhances vigorous early growth of the cane will enable it to outgrow the susceptible stage quickly.
Prompt application of irrigation was determined to be the most important agronomic tool in managing lesser cornstalk borer. Moist soil discourages female lesser cornstalk borer moths from laying eggs, and suppresses populations of larvae that are present in the crop. Field crews must wait a few weeks before installing irrigation lines in ratooned fields because rows are not evident until sprouts have emerged. The need to replant portions of ratooned fields can cause additional delays in installing drip lines. Deadhearts approached 100 percent in ratooned fields because of these delays in the late 1980s, when lesser cornstalkborer first became established in Hawaii. Extensive replanting of ratooned fields was required. One out of three ratooned acres had to be replanted at a cost of $300-400 per acre (A. Ota, unpublished data).
Primary shoots were and are heavily attacked by lesser cornstalk borer in fields where irrigation has not yet been applied. Typically the stand from ratooned fields is comprised of the secondary shoots that emerge after water has been applied. Stand development is therefore delayed a month or more for ratooned fields compared to planted fields. Don Heinz, then HSPA Director, wrote in his Newsletter for September 24, 1986, "…we recommend that newly planted or replanted fields be given highest priority for irrigation…" because of lesser cornstalk borer.
The parasitic wasp Orgilus elasmopalpi (Muesebeck) (Hymenoptera: Braconidae) was imported to Hawaii from Florida in 1986 for control of lesser cornstalk borer. Approval for release was obtained in June 1987. By 1990, more than 450,000 O. elasmopalpi had been released across sugarcane growing areas in Hawaii (Ota 1991). Few were ever recovered, and parasitism was usually measured at less than 1 percent. Pristomerus spinator (F.) is a braconid wasp that was introduced to Hawaii in 1942 to control armyworms. This parasitoid also attacks lesser cornstalk borer, and parasitism rates of approximately 4 percent have been measured. Orgilus elasmopalpi and P. spinator are considered important parasitoids of lesser cornstalk borer on the U.S. mainland.
In 1989, J.W. Smith of Texas A&M provided the parasitic wasp Horismenus elineatus (Schauf) (Hymenoptera: Eulophidae) to HSPA, where a colony was established. This parasitoid is originally from Bolivia. It is a gregarious parasite, which means that several wasps will emerge from one host larva. Chang and Ota estimated that up to 30 H. elineatus can emerge from each parasitized lesser cornstalk borer.
Permission for release of H. elineatus was received in 1990. Over 185,000 parasitoids were released in Hawaii that year (Ota 1991). The following year, more than 244,000 were released, a third of which were released on Maui (Ota 1992). In 1992, 30,000 were released, mostly on the island of Hawaii. Parasitism rates of up to 56 percent were measured on Kauai. The average rate of parasitism across the islands in 1992 was 25 percent (Ota 1993). Primary shoots are still heavily attacked by lesser cornstalk borer, even though H. elineatus is established in sugarcane growing areas. However, since the establishment of H. elineatus, ratooned fields recover when irrigation water is applied, and replanting is less frequent than when the lesser cornstalk borer first appeared. As of 2002, parasitism at the Hawaii Commercial & Sugar (HC&S) plantation on Maui averaged around 25%. Percent parasitism is slightly less at the Gay & Robinson (G&R) plantation on Kauai.
Horismenus elineatus continues to be recovered from fields more than 10 years after it was first released. Damage to ratooned fields has been reduced by H. elineatus, but the parasitoid has not eliminated the problem. It is still necessary to apply irrigation water promptly to emerging cane fields. It may be possible to increase levels of parasitism in ratooned cane through inundative releases of parasitoids - that is, by releasing several thousand into the field when the cane has reached a susceptible stage. However, it is unlikely that inundative releases will increase the level at which H. elineatus has become established on Maui or Kauai. Horismenus elineatus has ample host material in each location, and presumably has established some kind of equilibrium with its host and environment at its present levels. As the population density of lesser cornstalk borer fluctuates, so will the population of the parasitoid.
It is possible that the most effective and economical approach to reducing lesser cornstalk borer damage would be for HC&S and G&R to adopt a policy of prioritizing the irrigation of ratooned fields. Global positioning systems (GPS) and other components of precision agriculture could play a central role in improving the timing of irrigation in ratooned fields, if these technologies were adapted by the plantations. The investment of resources required to implement GPS must be considered in relation to the many possible benefits of such a system, particularly given the interest of the plantations in shifting to billeted (one-year) cane. Overhead irrigation systems that can apply water to fields once harvest is completed but prior to the emergence of ratooned shoots may also help suppress populations of lesser cornstalk borer.
Chang, V., and A.K. Ota. 1987. The lesser cornstalk borer: a new important pest of young sugarcane, pp. 27-30 in Annual Report, 1986. Experiment station. Hawaiian Sugar Planters' Association.
Chang, V., and A.K. Ota. 1988. The lesser cornstalk borer: a new important pest of young sugarcane, pp. 33-34 in Annual Report, 1987. Experiment station. Hawaiian Sugar Planters' Association.
Chang, V., and A.K. Ota. 1989. The lesser cornstalk borer and its control, pp. 26-29 in Annual Report, 1988. Experiment station. Hawaiian Sugar Planters' Association.
Chang, V., and A.K. Ota. 1990. The lesser cornstalk borer and its control, pp. 34-37 in Annual Report, 1989. Experiment station. Hawaiian Sugar Planters' Association.
Ota, A.K. 1991. Biological control of the lesser cornstalk borer, p. 32 in Annual Report, 1990. Experiment station. Hawaiian Sugar Planters' Association.
Ota, A.K. 1992. Bolivian parasitoid of LCB established in sugarcane fields, pp. 22-23 in Annual Report, 1991. Experiment station. Hawaiian Sugar Planters' Association.
Ota, A.K. 1993. Biological control of the lesser cornstalk borer, p. 25 in Annual Report, 1992. Experiment station. Hawaiian Sugar Planters' Association.