The two-spotted spider mite (Tetranychus urticae) is a significant pest that affects a wide variety of plants, including vegetables, fruits, ornamentals, and trees. Known for its destructive feeding habits, this mite can cause severe damage to plants if left uncontrolled. Below is an overview of its biology, symptoms, and management strategies.

Key Features

1. Physical Characteristics:

   • Oval-shaped, about 1/50 inch (0.5 mm) in length.

   • Common coloration: greenish-yellow, green, or translucent. Overwintering females are orange to orange-red.

   • Females (~0.4 mm) have elliptical bodies with 12 pairs of dorsal setae, while males are smaller with a tapered caudal end.

   • Dark spots (accumulated body waste) visible through the transparent body wall.

2. Life Cycle:

   • Composed of six stages: egg, larva, protonymph, deutonymph, and adult.

   • Eggs are laid on fine silk webbing and hatch in approximately three days.

   • Development time from egg to adult varies with temperature:

     • At optimal conditions (27°C), development takes 5–20 days.

   • Adult females live 2–4 weeks and can lay several hundred eggs during their lifespan.

   • Multiple overlapping generations occur yearly. Low soil moisture as well as temperatures 27°C promotes two spotted spider mite development.

Damage to Plants

1. Feeding Behavior:

   • Mites feed by piercing plant tissues with their needle-like mouthparts, primarily targeting the undersides of leaves.

   • Each feeding site destroys 18–22 plant cells per minute.

2. Symptoms:

   • Initial damage: Graying or yellowing of leaves.

   • Advanced damage: Formation of necrotic spots, stippled-bleached effects, and eventual yellowing, graying, or bronzing of leaves.

   • Severe infestations: Leaf drop and complete defoliation.

   • Flower damage: Browning and withering of petals, often mistaken for spray burn.

3. Economic Importance:

   • Infests over 200 plant species, including:

     • Ornamental Plants: Arborvitae, azalea, rose, viburnum, and more.

     • Fruit Crops: Blackberries, blueberries, and strawberries.

     • Vegetables: Tomatoes, eggplants, cucumbers, squash, and others.

     • Trees: Maple, elm, ash, redbud, and poplar.

   • Common in greenhouses and field-grown crops like chrysanthemums.

Detection and Monitoring

1. Observation:

   • Inspect undersides of leaves for mites, cast skins, and webbing.

   • Use a 10X to 15X magnifying glass for accurate detection.

2. Paper Test:

   • Place a sheet of white paper beneath the leaves and tap the foliage sharply. Mites falling onto the paper are more visible than on green foliage.

Management Strategies

1. Cultural Practices:

   • Maintain healthy plants to reduce susceptibility to mite infestations.

   • Remove and destroy infested plant parts to limit mite populations.

2. Biological Control:

   • Introduce predatory mites like Phytoseiulus persimilis and other natural enemies.

3. Chemical Control:

   • Use miticides judiciously to prevent resistance development.

   • Rotate chemical groups to manage resistance effectively.

4. Environmental Management:

   • Avoid conditions that favor mites, such as high temperatures and low humidity.

     
     

The study on Tetranychus urticae (two-spotted spider mite) development and reproduction on peach leaves (G. H. Hale cultivar) revealed how temperature affects its life cycle and reproductive parameters. Here are the key findings:

1. Development at Different Temperatures:

   • No development occurred at 13°C.
   • The egg-to-adult developmental time decreased with increasing temperatures from 17°C to 27°C but increased from 27°C to 33°C.
   • The total degree-days required for development above the lower threshold (13.79°C) was 136.43.

2. Egg Production:

   • The mean total egg production per female varied by temperature:
     • 25°C: 40.09 eggs
     • 27°C: 18.74 eggs
     • 30°C: 8.03 eggs
     • 33°C: 21.33 eggs

3. Longevity of Females:

   • The mean longevity of females was as follows:
     • 25°C: 12.91 ± 1.65 days
     • 27°C: 5.92 ± 0.55 days
     • 30°C: 3.56 ± 0.54 days
     • 33°C: 6.53 ± 0.56 days

4. Intrinsic Rate of Increase (rm):

   • The highest intrinsic rate of increase (rm) was recorded at 27°C (0.213 day^-1), and the lowest was at 13°C, where no development occurred.

5. Net Reproductive Rate (R0):

   • The net reproductive rate (R0) was highest at 25°C (16.87) and lowest at 30°C (4.18).

6. Doubling Time (DT):

   • Doubling time varied significantly with temperature. The shortest doubling time occurred at 27°C, and the longest at 30°C.

Conclusion: T. urticae can develop and reproduce across a broad range of temperatures. The optimal temperatures for its development and reproduction appear to be between 27°C and 30°C, which provide the best conditions for population growth.

The two-spotted spider mite is a challenging pest due to its rapid reproduction and extensive host range. Early detection and an integrated pest management approach are essential to mitigate its impact on crops and plants.