Comprehensive Guide to Phylloxera

On March 5, 2019, news emerged of fresh phylloxera damage in vineyards in the Yarra Valley, Victoria, Australia. Phylloxera represents one of the most devastating pests in viticulture, capable of causing catastrophic damage to grapevines. In the past, this insect caused such severe damage in Europe that European grape varieties were feared to be on the verge of extinction.

This major disaster, known as the Great Phylloxera Crisis, is an essential topic that invariably comes up in wine studies. While phylloxera stands as the ultimate enemy of wine grape growers, attention has focused primarily on the magnitude of the damage it causes, leaving its actual nature relatively unknown. This article provides a detailed examination of phylloxera, with particular emphasis on its ecology.

History of Phylloxera Damage

To understand just how devastating a pest phylloxera has been, let us first briefly review the history of its damage.

Phylloxera is originally native to North America and was not naturally distributed in Europe. The tragic history began in the 1860s when phylloxera hitchhiked on grapevine saplings imported from the American East Coast to southern France via London.

The following chronology outlines the historical progression of phylloxera damage:

• 1854: Phylloxera discovered in America
• 1863: Unexplained grapevine deaths confirmed in Provence (later recognized as Europe's first phylloxera damage)
• 1868: Root-infesting type of phylloxera identified
• Over 700,000 hectares of vineyards subsequently affected in a short period
• 1874: Phylloxera infiltrated Germany through ornamental grapevines imported from America in Bonn, with damage spreading southward
• Phylloxera damage expanded annually across all of Europe, affecting 2.5 million hectares in France alone (production dropped from approximately 84 million liters before the crisis to about 23 million liters in 1875)
• Japan also experienced major outbreaks from the Meiji to Taisho periods, bringing the nation's grape cultivation to the brink of complete destruction

What Kind of Insect is Phylloxera?

Phylloxera (Japanese name: Grape Root Aphid) belongs to the family Phylloxeridae, genus Viteus vitifoliae, and is an insect approximately 1mm in length with the scientific name Daktulosphaira vitifoliae. While multiple endemic species exist, they are all collectively referred to as "phylloxera."

Phylloxera possesses a sharp, needle-like mouthpart structure similar to that of mosquitoes, which it uses to suck sap from grapevines. This commonly leads to the explanation that grapevines die from having their sap sucked out. While this explanation is not incorrect, it should be noted that it is not entirely accurate either.

Phylloxera Ecology

Phylloxera ecology is extremely complex. Because its Japanese name is Grape Root Aphid, it is often assumed to parasitize only the root portions of grapevines, but it actually parasitizes and damages leaves and shoots as well.

As this makes clear, phylloxera reproduction can be divided into two categories: underground and above-ground activities.

Underground Reproductive Activity

Underground phylloxera reproduction occurs through parthenogenesis. In other words, all phylloxera existing underground are female individuals. These individuals lay eggs on the surface of grape roots. A single egg-laying session produces approximately 100-150 eggs.

The larvae and adults that hatch from these eggs absorb nutrients from grape roots while continuing to reproduce for multiple generations.

Above-Ground Reproductive Activity

In contrast, above-ground reproductive activity differs from underground reproduction in that it occurs through sexual reproduction. Most female individuals that develop underground lack wings and never emerge above ground. However, some of these individuals acquire wings and emerge above ground, initiating above-ground ecological activity.

Winged female individuals move from underground to the surface and fly to grape leaves and shoots, where they first engage in asexual reproduction. This egg-laying produces female individuals as in the underground case, but additionally produces male individuals. Notably, male and female eggs differ in size, with female eggs being larger and male eggs smaller in comparison.

The males and females thus produced mate and lay fertilized eggs. The individuals born from these eggs form galls on grape leaves and lay eggs within them to continue generations. These surface-born individuals may also move underground and engage in asexual reproduction on roots.

The above-ground cycle lasts approximately 8-10 days, during which newly born individuals form their own galls and lay eggs.

Importantly, while underground reproduction involves no genetic variation due to asexual reproduction, above-ground reproduction involves genetic changes due to sexual reproduction. In other words, phylloxera undergoes constant genetic change by repeating reproduction both underground and above ground, enabling it to acquire adaptive capabilities for environmental changes.

Damage Caused by Phylloxera

As previously mentioned, phylloxera pierces the surface of grape roots or leaves with its sharp mouthparts to suck sap. This action causes gall-like structures to form on grape roots and leaves, but this occurs in progressive stages.

Underground juvenile individuals particularly target young roots for sap extraction. At this stage, grape roots remain in a pear-shaped swollen state. Roots in this condition can still perform their original functions and do not directly cause grapevine death. In fact, this condition can be observed even in American rootstock roots known for phylloxera resistance.

However, as roots mature, what was merely a swollen condition gradually transforms into gall-like formations. The danger increases with this progression. This occurs because deteriorating root conditions prevent adequate absorption of nutrients and moisture from the soil.

Furthermore, roots damaged by phylloxera become more susceptible to bacteria and pathogens present in the soil, leading to root decay and death. This ultimately results in the death of the entire grapevine.

Similarly, when galls form on leaf surfaces, the stomata in the leaves cannot function normally. When stomata cannot function properly, respiration and photosynthesis are adversely affected, gradually causing leaf discoloration and ultimately leaf death and defoliation. This prevents grapevines from conducting normal physiological activities, also leading to tree death.

However, leaf damage is generally not as severe as root damage.

Phylloxera damage is actually not caused by phylloxera alone, but is a complex phenomenon also brought about by bacteria and pathogens present in the soil.

Phylloxera Control Measures

Phylloxera control measures can be broadly divided into the following two types:

• Chemical control
• Use of American variety rootstocks

Chemical Control

Chemical use is strictly regulated in Germany. Permitted usage conditions in Germany are limited to once per year for vineyards and twice per year for grapevine nurseries. Meanwhile, few chemicals are effective against phylloxera, and most actually used contain a component called imidacloprid.

While imidacloprid is one of the few chemicals expected to be effective against phylloxera, its use has recently become more difficult. This is because imidacloprid belongs to the so-called neonicotinoid class of insecticides. Neonicotinoid use is considered to have significant impacts on honeybees, leading to increasingly strict regulations toward prohibition in recent years. In line with this trend, imidacloprid use for phylloxera control is also becoming unavailable.

Rootstock Control

As previously mentioned, phylloxera has particularly significant impacts on grape roots. Among grape varieties, the European grape variety Vitis vinifera has the characteristic of being relatively resistant to gall formation on leaves, but has virtually no immunity to phylloxera in the crucial root system, quickly becoming covered with galls when parasitized by phylloxera.

In contrast, American grape varieties such as Vitis riparia and Vitis rupestris, having originally coexisted with phylloxera, have acquired resistance to it. Opposite to the Vitis vinifera case, these varieties are prone to gall formation on leaf surfaces but rarely develop galls on roots and can survive even when parasitized by phylloxera.

Rootstock control involves grafting European grape varieties onto American grape varieties with these characteristics for phylloxera protection.

Why Are Rootstocks Effective?

Why then can Vitis riparia and Vitis rupestris species avoid significant impact even when parasitized by phylloxera?

We have already explained that phylloxera damage is caused by two factors:

1. Nutrient and moisture absorption disorders due to root gall formation
2. Infection by bacteria and pathogens due to root damage

Vitis riparia and Vitis rupestris species inherently possess characteristics that make gall formation difficult even when subjected to sap extraction by phylloxera, but simultaneously feature hard, corky cells surrounding the central portions where important root tissues are located.

This barrier layer prevents phylloxera mouthparts from reaching the important parts of roots. As a result, important root portions avoid phylloxera damage, preventing both gall formation and bacterial and pathogen invasion of vital organs.

In other words, the existence of this barrier layer represents the phylloxera resistance strategy acquired by American grape varieties.

Rootstock Control Is Not Absolutely Safe

Indeed, using American grape varieties as rootstocks has succeeded in suppressing phylloxera damage. However, phylloxera undergoes genetic crossbreeding through sexual reproduction during its reproductive process, and no one can guarantee it will not undergo adaptive mutations in the future. Should phylloxera undergo some mutation and become capable of penetrating the root barrier layers of Vitis riparia and Vitis rupestris species to reach root centers with its mouthparts, rootstock-based control would become meaningless at that point.

And should this occur, we have already lost the backup strategy of neonicotinoid chemical control.

Nearly 100 years have passed since Vitis riparia and Vitis rupestris species began being used as rootstocks. With this much time elapsed, we must recognize that phylloxera could undergo corresponding mutations at any time.

What Should Be Done for Phylloxera Control?

Once phylloxera invades a vineyard, complete elimination becomes extremely difficult. This is due to phylloxera's short reproductive cycle and the difficulty of implementing control measures because its habitat is in the soil.

Therefore, when considering phylloxera control, the following two approaches must be considered simultaneously:

1. Measures to protect grapevines even when phylloxera is present
2. Measures to prevent phylloxera proliferation (prevent expansion of infected areas)

Measure 1, protecting grapevines even when phylloxera is present, primarily involves rootstock use. By using resistant rootstocks, grapevines can be protected and harvests secured even if phylloxera exists in the vineyard.

Measure 2, preventing phylloxera proliferation, includes conducting thorough pre-quarantine, not increasing own-rooted Vitis vinifera trees, performing careful land preparation when utilizing fallow land, and not leaving uprooted grapevines in vineyards.

Regarding own-rooted vines in particular, cases are often observed where growers increase them due to personal preferences, but from a regional or national perspective, this represents extremely undesirable behavior for phylloxera control. In this regard, wine grape growers are required to regulate their actions from a broader perspective.

Summary

Complete eradication of phylloxera from land once infected is extremely difficult and should be considered virtually impossible. Therefore, when fresh damage occurs as in the case of Australia's Yarra Valley vineyards, efforts must focus on preventing further expansion of infected areas.

While implementing control measures after infection occurs is certainly necessary, grape growers must pay careful attention to avoiding the creation of environments conducive to phylloxera expansion even before such incidents occur.

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