Beginner

Scientific Name
Tetranychus urticae

Identification

• Use a 10- 14x magnifying hand lens to see mites properly.
• Overwintering adults are orange in colour, otherwise, adult mites are 0.3- 0.5 mm (1/100- 1/50 in.) and greenish yellow with two dark spots on the back.
• Nymphs are similar in appearance only smaller. 
• Eggs are clear and round.
• All stages occur predominantly on the lower leaf surface.
• Damage appears as flecking or stippling on the upper leaf surface.
• Damaged leaves develop a bronzed hue, fruit and sepals may become bronzed if populations are very high.
• Webbing present on underside of leaf.

Often Confused With
Thrips
Cyclamen mite

Period of Activity
Overwintering adults can be seen on older overwintering leaves next to the ground in early spring.  Eggs are laid on these leaves and populations can appear alarmingly high at this time.  As new growth develops and two-spotted spider mite eggs hatch, mites will disperse throughout the plant to new growth.  There are multiple generations per year. Populations can build up to damaging levels after bloom but before harvest in June-bearing varieties.

Scouting Notes
First, determine where mites are present in the field.  Watch for signs of flecking and stippling on the older, lower leaves.  Walk the field in a zigzag pattern, periodically stopping to pick a middle-aged leaf.  Scan the underside of the leaf with a hand lens for mites.

Once you have determined where mites are the biggest problem, collect a sample of 60 leaflets (one of the trifoliates that make up a leaf)  from 40- 60 random plants of the variety in question.  Take the leaflet from a fully expanded leaf, avoiding the lower, oldest leaf next to the ground.

Examine each leaflet and note the stages present: eggs, nymphs, adults.

There are two ways to estimate mite populations:

1. Using a binocular microscope count the number of mites on each leaflet.  Determine the average number of mites per leaf.
2. Using a hand lens, record the number of leaflets with one or more mites:
   1. 25% of the leaflets (15/60) infested corresponds to a population of 5 mites per leaflet.
   2. 50% of the leaflets (30/60) infested corresponds to a population of 20 mites per leaflet.

An important window for mite control closes about 3-10 days before harvest, depending on the product used. Scouts should be sure to provide growers with detailed information 2 weeks before harvest so controls can be applied if necessary.

Thresholds
Thresholds have been suggested for mite control on strawberries ranging from 5 to 20 mites per leaflet.  Consider these thresholds and apply miticide if damage is evident and populations increase from week to week.  Use the low threshold before harvest and the higher threshold after harvest or in non-bearing plants.

Advanced

Scientific Name
Tetranychus urticae

Two-spotted spider mites are sporadic pests of strawberries; damaging outbreaks do not occur every year, or on every farm.  Scouts should watch closely for these pests however, especially in the bloom to preharvest period, because outbreaks are not predictable.

Identification
Use a 10- 14x magnifying hand lens to see mites properly.  Overwintering adults are orange in colour.  Otherwise, adult mites are 0.3- 0.5 mm (1/100- 1/50 in.) and greenish yellow with two dark spots on the back. Nymphs are similar in appearance only smaller.  Eggs are clear and round.  All stages occur predominantly on the lower leaf surface.

Spider mites feed on plant sap causing white flecking or stippling that is visible on the upper leaf surface.  Leaves develop a bronze cast. Webbing is visible on the lower leaf surface if populations are high.  High populations during the green fruit stage also cause fruit and sepal bronzing.

Often Confused With
Thrips
Cyclamen mite
(Both can cause fruit bronzing.)

Biology
Female two-spotted spider mites overwinter in crop debris on the ground.  In spring they move to new growth and feed on leaves closest to the ground.  There are many generations per year; eggs, nymphs and adults are frequently present at the same time.  As populations build and leaves age or become damaged, mites move up the plant to newer, more succulent growth.  The time for one generation to develop, from egg to mature adult, ranges from five days at 24°C (75°F) to three weeks at 12°C (54°F).

Period of Activity
Overwintering adults can be seen on older overwintering leaves next to the ground in early spring.  Eggs are laid on these leaves and populations can appear alarmingly high at this time.  As new growth develops and two-spotted spider mite eggs hatch, the mites will disperse throughout the plant to new growth.  Populations can build up to damaging levels after bloom but before harvest in June-bearing varieties.

Scouting Notes
First, determine where mites are present in the field.  Watch for signs of flecking and stippling on the older, lower leaves.  Walk the field in a zigzag pattern, periodically stopping to pick a middle-aged leaf.  Scan the underside of the leaf with a hand lens for mites.

Once you have determined where mites are the biggest problem, collect a sample of 60 leaflets (one of the trifoliates that make up a leaf) from 40- 60 random plants of the variety in question.  Take the leaflet from a fully expanded leaf, avoiding the lower, oldest leaf next to the ground.  Examine each leaflet and note the stages present: eggs, nymphs, and/or adults.

There are two ways to estimate mite populations:

1. Using a binocular microscope count the number of mites on each leaflet.  Determine the average number of mites per leaf.
2. Using a hand lens, record the number of leaflets with one or more mites.
   1. 25% of the leaflets (15/60) infested corresponds to a population of 5 mites per leaflet.
   2. 50% of the leaflets (30/60) infested corresponds to a population of 20 mites per leaflet.

Whether or not a problem develops depends on the weather, abundance of natural enemies, crop management practices and timing of insecticide applications for other pests.  Some varieties of strawberries are very susceptible to spider mites.  Check varieties separately when scouting for mites.

An important window for mite control closes about 3-10 days before harvest, depending on the product used. Scouts should be sure to provide growers with detailed information 2 weeks before harvest so controls can be applied if necessary.

Thresholds
Thresholds have been suggested for mite control on strawberries ranging from 5 to 20 mites per leaflet.  Consider these thresholds and apply miticide if damage is evident and populations increase from week to week. 

Use the low threshold before harvest, or on plants suffering from other stresses.  Use the higher threshold after harvest or in non-bearing plants.

Management Notes

• Factors that affect mite populations include:
  • Cultivar: Some varieties are more susceptible to mites than others.  Sparkle and Jewel are reported to be very susceptible.  Examples of varieties where we have seen mite problems in the past include Annapolis and Mesabi and the day neutral Seascape.
  • Crop nutrition: Plant tissue high in nitrogen favours mite development.
  • Crop management: Irrigation can affect mite populations.  Drought stressed plants are more susceptible to injury and favour increased mite populations.  Management practices such as renovation have an effect.  Mowing leaves after harvest helps to reduce mite numbers.
  • Pesticide use: Certain pesticides are harmful to naturally occurring beneficial insects and mites.
  • Surrounding vegetation: Severe outbreaks on beans or apple orchards can spread to adjacent strawberry fields.
  • Weather: Hot dry weather is favourable for mite development.
• There are several miticides available for strawberries.  Timing depends on the miticide to be applied and the crop stage.  Miticides work best when applied in warm weather, because mites are more active and eggs are hatching more quickly.  In cool weather, mites do not feed or move as much and are less likely to pick up miticide residue.
• Early spring applications of contact miticides may not be effective because strawberry plants are rapidly growing in the spring and leaves that expand after the miticide is applied do not have adequate residue for control.  Systemic miticides are more appropriate in early spring.
• Miticides should not be applied until mites are moving from older overwintering leaves to newer growth.
• Successful treatment requires thorough spray coverage, particularly of the leaf undersurface.  Most currently registered miticides are not systemic: mites must come in contact with residues.  Excellent spray coverage, with lots of air to move the leaves around, is needed for good control.  Remember, mites are on the lower leaf surface.  Drop nozzles between the rows can be used to direct spray up into the beds.
• Mites thrive on succulent plant tissue high in nitrogen.  Avoid excessive use of nitrogen on strawberries, especially before harvest.
• There are several predatory insects and mites that occur naturally in strawberry fields and feed on two-spotted spider mites.
• Beneficial mites are available for purchase from a number of suppliers.  Beneficial mites must be introduced before large populations of mites develop, but after insecticides for tarnished plant bug have been applied.