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Title: Re-evaluation of sugarcane borer (Lepidoptera: Crambidae) bioeconomics in Louisiana

item White, William
item Viator, Ryan
item Dufrene, Edwis
item Dalley, Caleb
item Richard Jr, Edward
item Tew, Thomas

Submitted to: Crop Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/31/2008
Publication Date: 5/20/2008
Citation: White, W.H., Viator, R.P., Dufrene Jr, E.O., Dalley, C.D., Richard Jr, E.P., Tew, T.L. 2008. Re-evaluation of sugarcane borer (Lepidoptera: Crambidae) bioeconomics in Louisiana. Crop Protection. 27(9):1256-1261.

Interpretive Summary: Preventing damage by larvae of the sugarcane borer moth with insecticides represents a major cost input by Louisiana sugarcane farmers. The use of these insecticides represents an important environmental concern as well. The underpinnings or bioeconomics (the relationship between pest numbers, crop response to injury, and resultant economic losses) of the current integrated control scheme were established more than 40 years ago and have not been evaluated since that time. We conducted this field study to re-visit the bioeconomics of sugarcane borer control in Louisiana using more accurate methods of determining yield and yield losses, current varieties, and current production costs. We found that many of the observations made 40 years ago remain valid today. For example, when all things are equal (ie. age of crop) no one variety is preferred over others by female moths as a place to lay their eggs. Also, some varieties are more susceptible to caterpillar entry into the stalks. However, the current varieties are more tolerant to damage by the caterpillars than those grown 40 years ago. That is to say they can sustain higher levels of injury yet still make high yields. These findings are of importance to farmers as they are increasingly seeing their profit margins diminish and therefore must save costs whenever possible. Data such as those reported in the current study allow farmers to better manage their pest problems thereby saving money and reducing the amount of insecticides going out into the environment.

Technical Abstract: The sugarcane borer, Diatraea saccharalis (F.) (Lepidoptera: Crambidae), is the key insect pest of sugarcane, Saccharum spp., grown in Louisiana. For more than 40 years Louisiana sugarcane farmers have used a value of 10% internodes bored at harvest as the Economic Damage level (ED) because damage lower than this value is not considered sufficient to cover insecticide costs. To remain below the 10% ED level, an Economic Threshold (ET) of 5% infested stalks has been established as an indicator of a need for insecticide applications. Three plant-cane studies were conducted to re-evaluate the long standing sugarcane borer ED level using the most recently released varieties of sugarcane. Varieties were exposed to artificially enhanced borer infestations; the experimental treatments consisted of borer control with insecticides or no control. Data were collected on infestation intensity, damage intensity, and associated yield losses. Crop yields from plots were obtained by mechanical harvesting and losses were classified as field losses e.g. losses of gross tonnage in the field and factory losses e.g. losses that were realized at the factory as cane is being milled. Factory losses are associated with decreased purity and cane quality. Farm income is based on the product of these two measures of yield, i.e. cane yield X sugar yield. In our study, seasonal stalk-infestation counts did not reveal any indication of preference by the borer moths for a specific variety; infestation pressure was generally uniform within a season among the varieties that we planted. Significant differences were detected among the varieties for harvest percentage of internodes bored as well as yields between borer controlled and non-controlled plots (P<0.05). In general, varieties were less susceptible to losses in the field (sugarcane yields) than in the factory (sugar yields). As a group, the most recent varieties released to Louisiana growers exhibit more tolerance to the borer than varieties grown 40 years ago. The percent reduction in sugar/ha loss per one percent internodes bored has decreased from an average of 0.74 for varieties grown in the 1960s to 0.61 as a mean for the newly released varieties. Although the cost associated with an insecticide application for sugarcane borer control has increased nearly 4-fold from 1971 to present, sugar yields have increased approximately 60% allowing farmers to offset some of these increased costs. Our economic analysis indicate that the ED level of 10% internodes bored is still relevant as a benchmark as the most at risk farmer, the tenant farmer, can afford to make at least one insecticide application to prevent season-long borer damage from exceeding 10% internodes. The planting of varieties resistant by either impeding larvae entry into the stalk or by increased tolerance to borer feeding, can have a profound effect on the economics of borer control by increasing the ED beyond 10% internodes bored, or by increasing the ET beyond 5% of stalks sampled infested with borer larvae.