When Beetles Attack: X. arvicola in the Vineyards of La Rioja, Spain

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One of the major problems to deal with in a vineyard is pests.  Different types of pests can cause an inordinate amount of damage to a vineyard, from reducing yield to lowering fruit quality and finally to plant death.  Depending upon where the vineyard is located, there are laundry lists of pests to avoid in nearly every corner of the world.  At times, pest species are known to cross over from invading one particular species to another, particularly in times of changing climate when some plant species are much more vulnerable and susceptible to attack.  One such example is the Xylotrechus arvicola beetle, a wood-borer that has been transported all over the world through the wood trade.

There are at least 26 species of beetle that have been reported to be pests of European vineyards, with

X. arvicola making it’s recorded grand entrance in the later 1970s.  X. arvicola is a beetle that is native to Spain that preys upon different woody species.  In La Rioja starting in the late 1970s, X. arvicola larvae were discovered inside the woody parts of the grapevines around the time of pruning, particularly those grapevines that were located close to rivers.

In the early 1990s, La Rioja saw a huge resurgence of the X. arvicola beetle, with the outbreak spreading to vines far away from adjacent forests.  It is not currently known what caused this outbreak, though it is speculated that it may have been a result of pesticide use that subsequently wiped out the predator of the X. arvicola beetle, allowing it to propagate rapidly with nothing to keep the population in check.

Identifying an X. arvicola-invested grapevine is pretty straightforward at a certain point, as the woody portions of the plant become riddled with adult “exit holes” that are about 4mm in diameter, has rachitic shoots, and results in lower yield at harvest time.  The exit holes are caused by the adults emerging from the woody parts of the grapevine after developing from the larval stage that had been embedded in this material.  It is not confirmed, but it is speculated that these beetles may be associated with various grapevine trunk diseases, but this research has yet to be completed.

In addition to La Rioja, X. arvicola is also a known pest in other winemaking regions of Spain, including Castilla y León, Castilla-La Mancha, Navarra, and Aragón.  In 2010, a study came out indicating that the percented of X. arvicola-infested grape vines in La Rioja grew to a whopping 94% in a 9 year time span, with the number of dead grapevines varying during that time between 2 and 17%.

The entire life cycle of X. arvicola is completed within the woody parts of the grapevine (or other plant species it attacks) and goes through a 2 year long development process.  Some studies have found bark removal treatments to be relatively effective in reducing the number of egg laying events into the grapevines, though it is noted that this type of treatment would be incredibly expensive and may not be

worth it in the long run.  Not much else is known about the life cycle of these insects, particularly after the emergence period into adulthood, so getting a grasp on the development of X. arvicola is critical to developing a way to combat against the pest in the future.

The purpose of the study presented to you today was to observe flight patterns for X. arvicola, as well as applying this information to a mathematical model in order to have more accurate information regarding the adult and sexually-mature part of the lifecycle, which then could be used to develop techniques or treatments to prevent damaged caused by X. arvicola.

Methods

5kg of invested Vitis vinifera (var. Tempranillo) were left in plastic containers in a field in the village of Tirgo, La Rioja, Spain, from March 1^st to August 30^th.

Emergence of adults was monitored and recorded on a weekly basis.

Climate/meteorological information was collected from a weather station at Cuzcurrita de Río Tirón, which was located about 1km from the plastic bins containing the infested grapevines.

This observational survey was conducted over 6 years, between 2003 and 2008.

Degree-day data was plotted against male, female, and total emergence.

Results

• Adult emergence at the site started in the end of May through mid-August.
• The number of X. arvicola adults per kg of grapevine woody material ranged from 0.94 in 2004 to 1.82 in 2008 (6 year average: 1.35 adults/kg).
• Males emerged 1 to 4 weeks earlier than females.
• The first females emerged in mid-June.
• In June, males outnumbered females, while the ratios were nearly equal by July.
• Total emergence (both male and females) showed a single peak sometime between late June and mid-July, indicating that in this region of Spain (or at least these particular broods), X. arvicola adults have only one main emergence period during a season.

Conclusions

The overall goal of this study was relatively simple in that it sought to determine the emergence behavior of both male and female X. arvicola beetles in La Rioja, Spain, a species that in recent decades has become a more prominent pest in this region of the world.  In order to even begin thinking about how to combat such an insect pest, one must first understand its life cycle.  From larvae to adult, as well as natural enemies, collecting some basic entomological data is a critical first step in the development of a treatment plan.

According to the results of this study, over a 6 year period, there appeared to be a single main emergence period for these beetles (fancy science term = monovoltine), with males emerging a few weeks before females.  On the contrary, another study in a different part of Spain found 2 separate peaks (multivoltine), indicating that the emergence behavior of the same species may be different depending upon the local microclimate, though many more studies are required to determine if this is the case, or if one (or both) of the studies had insufficient data.

Assuming the results of this study are correct for the sake of argument (though I think the study should be

repeated by other labs just to be sure), it appears as though the optimal timing for treating adult X. arvicola would be between mid-June through the end of July.  At the pruning stage, infected woody parts could also be removed, decreasing the potential number of adults the following summer.

Before coming right out with a treatment, however, it would be worth understanding who the natural predators of X. arvicola are. Is there something else going on that is causing an unnatural drop in the natural predator populations of X. arvicola?  Could we somehow utilize these natural predators into a vineyard management program to attempt to rid the investation without needing to resort to chemical means?

Also, does X. arvicola have a “preference” for one particular grapevine varieties over another?  In other words, are there any particular varieties that are more prone to X. arvicola attack than others?

It would also be interesting to know if X. arvicola infestations are related in any way to wood trunk diseases of the grapevine.  Does one cause the other? Do we have a chicken and the egg scenario here?

A lot of important questions can be raised from this basic life cycle study of X. arvicola beetles, which may lead to more research and advances in how to combat against these pests, which if left untreated could seriously harm the viticulture and economy of the La Rioja region.

What do you all this of this study?  Please feel free to leave any and all comments!

Source: Soria, F.J., López, M.A., Pérez, M.A., Maistrello, L., Armendáriz, I., and Ocete, R. 2013. Predictive model for the emergence of Xylotrechus arvicola (Coleoptera: Cerambycidae) in La Rioja vineyards (Spain). Vitis 52(2): 91-96.