Phylloxera

What is phylloxera?

Phylloxera refers to a genus of insects (Phylloxera, order Homoptera) that are primarily North American and act as serious plant pests. While many species in this genus cause damage to trees like hickory and pecan, the most notable is the grape phylloxera (Phylloxera vitifoliae).¹²

This small, greenish-yellow insect is highly destructive to grapevines. By sucking fluid from the plant, it creates galls on leaves and nodules on roots, eventually causing the plant to rot. In the mid-19th century, grape phylloxera was introduced to Europe from the eastern United States, devastating the wine industries in France, Germany, and Italy. It also threatened vineyards globally between 1870 and 1900.²³

The industry was saved by grafting European wine grapes onto rootstalks of vines native to the eastern United States, which are naturally immune to the pest. Other control methods include the use of hybrids and fumigants.²³

How does grafting protect grapevines from pests?

Grafting protects grapevines from pests like grape phylloxera by utilizing the natural biological defenses of specific plant varieties.

The process involves taking a segment of a stem from a desired fruiting variety—such as the European wine grape—and attaching it to a rootstock cutting that is native to the eastern United States. While the European vines are highly susceptible to the root-destroying nodules caused by phylloxera, the American species possess a natural immunity to the pest.¹⁴

By combining these two parts, growers can cultivate high-quality fruit on a root system that remains resistant to infestation, effectively preventing the vine from rotting. To ensure the integrity of this protection, the point of union between the scion (the fruiting variety) and the rootstock must be kept well above the ground to prevent the scion from developing its own roots, which would be vulnerable to the insects.¹⁴

What are other examples of invasive species affecting agriculture?

Invasive species often disrupt agricultural systems by outcompeting native organisms, spreading diseases, or destroying crops when they are introduced to new environments without their natural predators. Several notable examples have had significant economic impacts:⁵⁶

• Colorado Potato Beetle (Leptinotarsa decemlineata): Originally a native insect of semidesert regions feeding on the buffalo burr plant, it adapted to the potato after the crop was introduced to the Colorado region in the 1840s. It subsequently spread across the United States and eventually to Europe, causing severe destruction to potato crops.⁷⁸
• Emerald Ash Borer: Arriving in the United States in the 1990s, likely via shipping materials from eastern Asia, this pest has been responsible for the deaths of hundreds of millions of ash trees in North America.⁵
• European Corn Borer (Ostrinia nubilalis): Accidentally introduced to the United States, this pest became a major threat to corn production by feeding on the plant and escaping its natural range of enemies.⁷⁸
• Cottony-Cushion Scale (Icerya purchasi): This scale insect threatened the California citrus industry in the late 19th century until it was controlled by the introduction of the vedalia beetle from Australiavedalia beetle](https://www.britannica.com/animal/vedalia-beetle) from Australia.⁵⁷⁸
• Chestnut Blight: This fungal disease, introduced from abroad, practically annihilated the American chestnut tree.⁸

These examples highlight the risks posed by global commerce and the movement of goods, which often facilitate the spread of exotic pests into new habitats.⁵⁶

What is the difference between parthenogenetic and sexual reproduction in insects?

In insects, the primary distinction between sexual reproduction and parthenogenesis lies in the requirement for fertilization.

Sexual Reproduction

In the vast majority of insects, reproduction is sexual, requiring the union of male and female gametes (sex cells) through fertilization. This process typically involves complex behaviors to bring mates together, such as the use of pheromones, visual displays, or acoustic signals like the chirping of crickets. Sexual reproduction is essential for genetic recombination, which often produces offspring better equipped to survive adverse environmental conditions, such as winter cold.¹¹¹²¹³

Parthenogenesis

Parthenogenesis is a reproductive strategy where an egg develops into an individual without being fertilized by a male gamete.¹²¹³

• Mechanisms: It can occur through apomixis, where offspring are genetic clones of the parent, or through processes that involve meiosis, resulting in genetic variation.¹²
• Forms: In the order Hymenoptera (bees, ants, wasps), it can produce haploid males (arrhenotoky) or females (thelytoky).¹²
• Cyclic Patterns: Many insects, such as aphids, exhibit heterogony, alternating between parthenogenetic summer generations and sexual winter generations in response to environmental cues like temperature or food availability.¹²¹³¹¹