Wednesday, October 22, 2014

Biological Control of Hemlock Woolly Adelgid

hemlock woolly adelgid
The hemlock woolly adelgid (Adelges tsugae), a native of Asia, is a <1 mm long reddish purple insect that lives within its own protective coating. White, woolly masses that shelter these sap-feeding insects can be found at the bases of hemlock needles along infested branches. The presence of these white sacs, which resemble tiny cotton balls, indicate that a tree is infested. The hemlock woolly adelgid is a threat to North American hemlock forests. As of 2007, 50% of the geographic range of eastern hemlock (Tsuga canadensis) had been impacted. The feeding activities of these hemipterans reduces new shoot growth, premature needle drop, thinned crowns, branch tip dieback, and eventual tree death. 

Laricobius osakensis
Aside from recommending the use of insecticides researchers focused on biological control measures for this pest. In 2006 the derodontid Laricobius osakensis was imported from Japan to the United States for study in quarantine facilities as a potential biological control agent. Four years later it was granted release from quarantine by the U.S. Department of Agriculture. However, it seems that this was premature. A new study published in the Southeastern Naturalist describes what happened not much later:

However, after sequencing DNA barcodes for members of the L. osakensis colony in the fall of 2011, it was discovered that the colony was contaminated by another Japanese species, Laricobius naganoensis. 

The problem is that regulations clearly state that insects shipped from abroad must not contain unauthorized species; therefore the presence of L. naganoensis within the L. osakensis colony resulted in the placement of the L. osakensis colony back into quarantine before beetles were released in the field.

Laricobius naganoensis is a species that lives in sympatry with Laricobius osakensis. Both species are morphologically very similar which makes it difficult to differentiate them. Females cannot be reliably differentiated using morphology and males only by their genitalia but this identification requires dissection of dead specimens which is something one would like to avoid if the goal is to release the beetles as biological control agent.

The researchers were looking for  a quick and inexpensive assay to identify both species and evaluated non-lethal DNA extraction methods. They designed a restriction fragment length polymorphism (RFLP) assay based on two enzymes (AluI and MboII ) using DNA Barcodes. In addition they found out that a single antenna from a specimen is sufficient to retrieve a DNA Barcode sequence. Further research will have to show if the removal of an antenna will have an impact on beetle survival and reproduction but this is a very promising result:

Without the proper permits, L. naganoensis cannot be released legally in the US. Therefore, distinguishing between L. osakensis and L. naganoensis is currently necessary for universities and state or federal agencies that will be importing L. osakensis from Japan for biological control of Hemlock Woolly Adelgid. The RFLP assay developed here is less expensive and less time consuming than DNA sequencing, and the equipment needed for this assay is available in most basic molecular labs. The enzymes AluI and MboII were each sufficient for distinguishing the species. However, since there is likely to be more natural diversity than we have sampled to date, possibly resulting in additional banding patterns, we recommend using both enzymes independently and sequencing any individuals for which the assay results do not match or which produce new gel patterns not reported here.

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