The Sugarcane Borer

The sugarcane borer moth, Diatraea saccharalis, is widespread throughout the Western Hemisphere, and is considered an introduced species in the southern United States. Although this moth has a wide distribution and is a pest of many crop plants including sugarcane, corn, sorghum and rice, it is considered one species.

The larvae bore into the sugarcane stalks. In mature plants the tops tend to weaken or die, sometimes breaking off. In young plants the inner whorl of leaves is killed, resulting in a condition known as "dead heart." The amount and purity of juice that can be extracted from cane is reduced when borers are present, and sucrose yield may be decreased 10 to 20%. Lastly, when seed cane is attacked, the tunneling by borers makes the seed piece susceptible to fungal infection. Sugarcane borer attacks plants in the family Poaceae (true grasses). Though principally a pest of sugarcane, this insect also will feed on other crops such as corn, rice, sorghum, and sudangrass. However, the damage to those is usually fairly modest.

Despite the damage caused by this pest species only few studies have investigated the existence of cryptic species or the population structure of this moth. Especially for species with a widespread distribution it should be determined whether they are truly one species, or rather a complex of sibling species. In addition, it is unknown if this insect may have been introduced into the southern United States once or on several occasions which would be reflected in genetically distinct populations which can vary in their susceptibility to natural enemies and control measures. 

New research, just published in PLoSONE, focused on the population structure of  sugarcane borers in the southern United States with the hopes to contribute to its management as well as to help identify future introductions and their likely region of origin. A group of US researchers investigated this question by collecting D. saccharalis in Texas, Louisiana and Florida and by examining their population structure using amplified fragment length polymorphisms (AFLPs). In addition, a 658 base pair region of the mitochondrial DNA COI gene was sequenced from several individuals from each southern United States population. The mitochondrial COI sequences were compared to publicly available COI sequences for D. saccharalis, to investigate potential source populations for those established in the southern US, as well as to estimate the number of potential cryptic species which may exist within this species.

The DNA Barcode data of the study indicated the existence of at least three distinct lineages: A Florida lineage, a lineage including Texas, Louisiana and Mexico, and a third lineage from South America that includes Brazil, Argentina, and Bolivia. In the literature a fourth lineage from Colombia is also discussed. Both AFLP and COI analysis show that the Florida lineage represents a genetically distinct cluster. This degree of genetic divergence suggests that Florida D. saccharalis could represent a distinct species.

The authors make clear that it would be premature to speak of several new cryptic species:

To be robust, defining species limits should include multiple lines of evidence. Such an approach is referred to as integrative taxonomy and should include morphological, behavioral, molecular and geographic data. Thus, although our data strongly suggests the existence of a D. saccharalis cryptic species complex, further lines of evidence would provide additional support of this assertion.

However, even if we don't know if we are looking at several distinct species or not, the study represents a big step forward:  Genetically distinct lineages may differ in their damage potential and/or in their vulnerability to pest control strategies such as biological control. The ability to characterize and identify genotypes of D. saccharalis and related species or as of yet undiscovered species will improve pest management efforts against this pest and improve area-wide control efforts across its geographic distribution.