Common Pests: Aphids, Western Flower Thrips, and Intestinal Worms

Overview

The VCAA study design specifically names these three pests for detailed study. Each causes significant economic losses in Victorian agriculture and horticulture and requires targeted management strategies. Understanding their biology (lifecycle, biology) is essential for effective control.

VCAA FOCUS: Know the classification (metazoal), biology/lifecycle, type of damage caused, and prevention/control strategies for each of these three pests.

Pest 1: Aphids

Classification

• Category: Metazoal pest
• Order: Hemiptera (true bugs)
• Common species: Green peach aphid (Myzus persicae), cabbage aphid (Brevicoryne brassicae), cotton aphid (Aphis gossypii)
• Host range: Very broad — vegetables, fruit trees, cereals, ornamentals

Biology and Lifecycle

Aphids have a complex lifecycle that includes both asexual (parthenogenesis) and sexual reproduction:

1. Wingless females (apterae) reproduce parthenogenetically (without mating) through spring and summer — rapid population build-up
2. Under stress (overcrowding, temperature change), winged forms (alates) develop and disperse to new host plants
3. In cooler climates, sexual reproduction occurs in autumn, producing overwintering eggs
4. In mild climates (e.g., Victoria’s coastal areas), populations can persist year-round

Reproduction rate: Under optimal conditions, a single aphid can produce up to 50–100 offspring per week — populations can explode rapidly.

Damage

Direct damage:
- Piercing and sucking sap from phloem vessels — causes wilting, stunting, and leaf distortion (curling)
- High populations can cause branch dieback in fruit trees

Indirect damage (often more significant):
- Honeydew excretion — sticky substrate promotes growth of sooty mould fungus, reducing photosynthesis and marketability
- Virus transmission — green peach aphid is the most important vector of plant viruses in Australian horticulture (e.g., cucumber mosaic virus, potato virus Y)
- Even brief aphid feeding can inoculate a plant with a virus before the insect is killed by a pesticide

Prevention and Control

EXAM TIP: For virus-transmitting aphids, chemical control is often ineffective at preventing virus spread because the aphid transmits the virus before dying from the insecticide (non-persistent transmission). Integrated approaches using reflective mulch and resistant varieties are more effective for virus management.

Pest 2: Western Flower Thrips (Frankliniella occidentalis)

Classification

• Category: Metazoal pest
• Order: Thysanoptera
• Origin: Native to western USA; accidentally introduced to Australia in the 1990s via ornamental plant imports
• Host range: Over 250 plant species — vegetables, ornamentals, fruit crops

Biology and Lifecycle

• Lifecycle completes in ~10 days at 25°C; faster in warm conditions
• Adults and larvae feed by rasping plant cells and sucking contents

Damage

Direct feeding damage:
- Silver streaking and scarring on leaves, petals, and fruit — cosmetic damage affecting marketability
- Distorted, crinkled leaves and flowers
- Particularly damaging to seedlings and soft growing points

Indirect damage (highly significant):
- Primary vector of Tomato Spotted Wilt Virus (TSWV) — one of the most damaging plant diseases in Australian horticulture
- Even low thrips numbers can cause unacceptable virus spread
- TSWV causes necrotic rings, stunting, and death in tomatoes, capsicums, lettuce, and many other crops

Prevention and Control

COMMON MISTAKE: Students sometimes focus only on direct feeding damage. The real economic impact of WFT in many crops is virus transmission (TSWV) — even one infected adult can spread virus to many plants before being controlled.

Pest 3: Intestinal Worms (Gastrointestinal Nematodes)

Classification

• Category: Metazoal pest (endoparasites)
• Phylum: Nematoda
• Key species:
• Haemonchus contortus (Barber’s Pole Worm) — sheep, goats; blood-sucker; most economically damaging
• Trichostrongylus spp. — sheep, cattle; damages intestinal lining
• Ostertagia spp. — cattle; inhabits abomasum

Biology and Lifecycle (using Haemonchus as example)

$$\text{Eggs (in dung)} \rightarrow \text{L1 larvae} \rightarrow \text{L2 larvae} \rightarrow \text{L3 (infective)} \rightarrow \text{Ingested by host} \rightarrow \text{L4} \rightarrow \text{Adult worm}$$

Key features:
- L3 (infective larvae) — the stage that survives on pasture and is ingested by grazing animals
- Cycle completes in ~20 days under warm, moist conditions (Haemonchus)
- A single female Haemonchus worm can lay 5,000–10,000 eggs per day
- L3 larvae can survive on pasture for months in cool, moist conditions; killed rapidly by hot, dry weather

Damage and Symptoms

Haemonchus: Blood-sucking — each adult worm removes ~0.05 mL blood/day; heavy burdens cause severe anaemia, oedema (bottle jaw — fluid under chin), lethargy, and death.

Prevention and Control

KEY TAKEAWAY: Drench resistance in intestinal worms is a major crisis in Australia — over-use of anthelmintics has selected for resistant populations. The solution is strategic targeted treatment using FECs, FAMACHA, refugia, and rotation of drench classes. This is a core concept linking to the biological resistance KK.

Summary Comparison