Monday, August 11, 2014

2 x Bemisia tabaci

The silverleaf whitefly (Bemisia tabaci) is one of several whiteflies that are currently important agricultural pests.Their nymphs use their mouth parts to stab into the plant and consume the plant’s juices. The honeydew they leave behind can induce the growth of sooty molds, which can then reduce the plants ability to absorb light. This results in less growth, lower yield, and poor quality plants. It is thought that the United States alone has suffered crop and ornamental plant damages in excess of $1 billion through this pest. All over North America, Bemisia tabaci is a particularly important pest of greenhouse poinsettia (Euphorbia pulcherrima) .

Actually, Bemisia tabaci is a cryptic species complex consisting of at least 24 morphologically indistinguishable yet behaviorally and physiologically distinct species. The currently accepted classification represents the presumed phylogeographic origins of the proposed cryptic species and is based on a partial COI sequence derived from the 3′ region of the gene. This is not the accepted DNA Barcode region which is derived from the 5' region from the same gene. The authors of the first of two papers I am presenting in this post comment on this problem:

Given the widespread adoption of DNA barcoding as a tool for species identification, we would encourage researchers generating COI sequence data for B. tabaci to consider including both the 5′ and 3′ regions, and to follow DNA barcoding metadata standards. This will ensure that data collected with one marker can be unambiguously linked to data collected from the other and that the data will be useful to the largest community possible.

Two papers on Bemisia tabaci within a fairly short time frame are proof on how important the issue is. Interestingly both studies were done at our institute but they are completely independent from each other focusing on different regions of the planet. 

Paper number one looks at the situation in Canadian Greenhouses and more specifically at the poinsettia cultures therein. In local poinsettia production, two species from the Bemisia tabaci complex, Mediterranean and Middle East Minor 1, often infest crops simultaneously. Differences in pesticide susceptibility between these two cryptic species have the potential to influence growers' management decisions, including the use of biological control or insecticides, and the choice of insecticide active ingredient.

The colleagues did a survey of populations in commercial greenhouses here in Ontario, Canada, and they were able to show that under biological control, the species Middle East Minor 1 can displace the species Mediterranean within one growing season (5-6 fly generations), whereas under insecticide treatment Mediterranean numbers stay stable because this species is more resistant to, and likely has a greater capacity to develop resistance to, a range of insecticides. These results have important implications, e.g. the ability of Middle East Minor 1 Bemisia tabaci to naturally displace Mediterranean under biological control-based management may increase the efficacy of “clean-up” insecticide applications when necessary, which can be important for ornamental crops. These data also support the use of biological control-based Bemisia tabaci management, as it can effectively reduce Bemisia tabaci populations below economic levels while allowing Middle East Minor 1 to displace Mediterranean.

For study number two we have to travel to the other side of the planet. In Pakistan three species (Asia 1, Asia II 1, and again Middle East Minor 1) are associated with the transmission of cotton leaf curl disease which causes a significant reduction in yield. This disease is caused by a virus which devastated the Pakistan cotton industry in the early 1990s where it led to an estimated yield reduction of 30-35%. The severity of cotton leaf curl disease varies across Pakistan with higher losses in central (Punjab) than southern (Sindh) Pakistan. There has been a continuing debate as to the identity of the whitefly lineages in these regions and whether differences in the vector pool account for the differing levels of infection on cotton plants from these provinces. Because Asia II 1 was associated with a higher incidence of cotton leaf curl disease in both Punjab and northeastern India , it is thought to play an important role in the transmission of this disease.

This study used DNA Barcodes and the COI 3′ region and showed that six species of the Bemisia tabaci complex were present in Pakistan. The COI-3′ sequences were used to map five of them to known species but the authors also found a new species which they named “Pakistan”. Concerning is the fact that the species Asia II 1, previously restricted to Punjab, is now the dominant lineage in southern Sindh. This expansion to the south may have serious implications for cotton plantations in this region. 

Both studies tackle the problem of COI-3' vs. COI-5' (DNA Barcode) by sequencing both gene regions (either independently or by sequencing the entire COI gene) and thereby help to establish a translation matrix from COI-3′ to DNA Barcodes which will in turn help to build a much needed DNA Barcode reference library for this agglomerate of pest species.

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