Publication 360, Fruit Production Recommendations: Thresholds

Publciation 360, Fruit Production Recommendations > Chapter 2, Pest Management > Thresholds

Excerpt from Publication 360, Fruit Production Recommendations 2010-11,
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Table 2-3. Examples of Degree-Day Models Used in Fruit Crops

Introduction to Thresholds

An IPM program uses control guidelines or thresholds to decide when to apply pesticides to prevent economic loss. Some damage to the crop is tolerated as long as this damage does not exceed the cost of the control.

For insect pests, thresholds are usually based on the presence of the pest at certain levels. Thresholds for direct pests, which feed on fruit and have an immediate effect on fruit quality, are generally lower than thresholds for indirect pests that feed on leaves, stems or roots.

Disease guidelines may be based on damage potential. This is estimated with weather models, crop tolerance, stage of crop development and field observations.

Thresholds have not been developed or validated for all pests in Ontario. Even established thresholds require adjustment for different varieties, markets and crop vigour. Established thresholds may also require revision to optimize the use of new, reduced-risk products.

Use the thresholds in Table 2-2. Examples of Thresholds Used for Fruit Pests in Ontario, together with information on sampling techniques and sample size, crop growth stage, spray times and pesticide characteristics discussed in detail in each crop chapter.

**Table 2-2. Examples of Thresholds Used for Fruit Pests in Ontario** (PDF 117 kb)
Crop	Pest	Spray Timing	Minimum Sample Size	Threshold	Comments
Strawberries	Tarnished plant bug	Bloom-green fruit	20 flower and fruit clusters	Approx. 0.25 nymphs per cluster	Sequential sampling methods are preferred. See Tarnished Plant Bug on the OMAFRA website at www.ontario.ca/cropipm.
Strawberries	Clipper weevil	Before 1st bloom	5 locations	13 clipped buds per 2 ft² row	Sample outer rows.
Strawberries	Two-spotted spider mite	Before harvest, or July-August	50 leaflets	Low threshold: 5 mites per leaflet High threshold: 20 mites per leaflet	Use low threshold on sensitive varieties before bloom or when using beneficial mites for control. Use high threshold after harvest.
Raspberries	Raspberry crown borer	October, or when primocanes begin to grow in spring	Entire block	5% of canes with die back
Apples	Mullein bug	Petal fall to calyx	25 taps per block	7 to 9 nymphs per 25 taps	Sample susceptible varieties such as Red Delicious and Northern Spy.
Apples	Spotted tentiform leafminer	Pink to calyx	50 spurs	3 to 5 eggs per spur	Apply insecticide at first egg hatch.
Grapes	Potato leafhopper	Early summer after surrounding alfalfa hay is cut.	Entire block	In established blocks: 25% or more of young leaves with symptoms In 1st or 2nd year plantings: 10% of young leaves with symptoms	Apply insecticide when most of the population is in the nymphal stage. Symptoms include yellowing along the leaf edge and leaf curling.

Degree-day modelling and determining a biofix

Temperature, light and moisture affect the growth and development of plants and their pests. Of these, temperature is the most important factor. Insect and mite development is closely related to the daily accumulation of heat. These pests need a certain amount of heat, or energy, to move to the next development stage.

The amount of heat required for insect and mite development remains constant from year to year, but depending on weather conditions, the amount of actual time can vary. Insects and mites have a minimum and maximum base temperature below or above which development does not occur. These base temperatures are different for each organism.

Growing Degree-Days (GDD) are used to estimate the growth and development of pests in the growing season (see Table 2-3. Examples of Degree-Day Models Used in Fruit Crops, page 10). Events such as egg-laying, egg hatch, movement of crawlers or the occurrence of disease infection can be predicted and used to schedule inspection and spray programs. For example, degree-day calculations can predict the first hatch of codling moth eggs or the percentage of apple scab ascospores that have matured in the orchard.

There are several methods used to calculate GDD, but the method commonly used with simple monitoring equipment is the averaging method or "max/min" method. GDD for a given organism are calculated as follows:

GDD = ((Daily max°C) + (Daily min°C) ÷ 2) - minimum base temperature

Growing Degree-Days are accumulated daily until a pest-specific total is reached. The averaging method works well in most years. However, the actual GDD accumulations may be underestimated in extended periods of cool weather or overestimated in hot weather.

An example of the averaging method on a relatively cool spring day:

For a given pest:

Lower base temperature = 10°C

Upper base temperature = 35°C

On a given day:

Minimum temperature = 5°C

Maximum temperature = 15°C

Growing degree-days (GDD) for that day is = ((15+5) ÷ 2) - 10 = 0

Note that the maximum temperature was higher than the base temperature for the insect, so growth and development were possible for at least part of the day. However, no GDD were accumulated. This illustrates how cool temperatures, especially over several days, could lead to an underestimation of insect development.

GDD are either accumulated from a set start date, such as April 1, or from a specific event known as a biofix. A common biofix used for insects is the first sustained catch in pheromone traps.

Using a biofix provides predictions that are more accurate and requires tracking temperatures over a shorter period..

There are several limitations to degree-days models:

Factors such as humidity, light intensity and rainfall also affect pest development. As a result, GDD predictions are only estimates of pest development. Verify these predictions with field observations.
Temperatures used to determine GDD must represent the environment where organisms develop. Use weather data collected from within a mile or less of the actual orchard or field being monitored.
GDD have been developed and validated for only a few fruit pests in Ontario.

**Table 2-3. Examples of Degree-Day Models Used in Fruit Crops** (PDF 106 kb)
Pest	Model
Tarnished plant bug (strawberries)	· Degree-days are used to predict first nymphs in strawberries · 30-40 GDD (base 12.1°C) after April 1
Codling moth (apples)	· Degree-days are used to predict first egg hatch · 139 GDD ( base 10°C) after biofix (first sustained moth catch)

For a list of crop consultants that provide monitoring services, contact OMAFRA�s Agricultural Contact Centre at 1-877-424-1300.

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