Activity - Paper Bag Orchard

Apples are an important fruit crop that have high "cosmetic standards" that is, very little pest damage can be tolerated because customers want perfect apples!
Activity - Paper Bag Orchard - Articles


Content Objective(s):

Students will be able to:

  • Apply recognized scouting methods to sample insect populations.
  • Compare population densities with action thresholds.
  • Analyze and re-evaluate existing management programs.

Assessment Strategies:

  • Performance assessment
  • Correct results to calculations


Apples are an important fruit crop that have high "cosmetic standards" that is, very little pest damage can be tolerated because customers want perfect apples! This has resulted in growers using significant number of pesticide sprays on apples over the years. With many sprays of similar chemicals, pest populations can become resistant to the chemicals, requiring higher and higher doses to kill them and finally, the chemical no longer kills the pest. Clearly, Clearly, it is advantageous to understand the pest life cycle and be able to estimate its population growth so fewer, well-times sprays do the job.

In addition to pests in crops there are also beneficial insects. A beneficial insect is a predator or parasitoid that kills the pest insects. Manipulating beneficial insects to reduce pest damage is called biological control or biocontrol.

A third factor is the tree itself. Trees undergo stress from various factors. A stressed tree is more sensitive to pest damage than an unstressed tree. Sources of stress for trees include drought, poor soil, disease, time in the growing season, shade and crop load (how many apples there are).

In this activity, we will apply knowledge about pests and beneficial insects and tree stress to make an IPM decision for an apple orchard.

The European red mite (Panonychus ulmi) is a major pest of apple trees. The mite was introduced into North America from Europe in the early 1900s and is now established in most fruit growing areas. The tiny eight-legged female mite is 1/64 inch long and bright red. She can lay up to 35 eggs during her average life cycle of 18 days with eight to ten generations of this mite produced per year. The male is smaller, lighter in color, and has a pointed abdomen. The rate at which mites develop is primarily temperature dependent. Hot, dry weather favors development, while cool, wet weather delays mite activities.

Mite feeding injury to the tree leaves can be great with the most serious injury occurring in early summer when trees are producing fruit buds for the following season. Mites feeding on leaves also reduce the ability of leaves to manufacture enough food (carbohydrate) for desirable sizing of fruit. A characteristic brown foliage that, in severe cases, becomes bronze, results from heavy mite feeding.

In Pennsylvania, there is an effective mite predator, the black black ladybird beetle (Stethorus punctum). This beetle (a relative of the 'ladybug') is naturally found in Pennsylvania and spends the winter in the litter under the trees. In the spring the beetle emerges and looks for a place to lay eggs. A beetle will lay eggs only where mites are present. If enough of these beetles are in the orchard they will eat enough mites to keep the mites below the economic threshold.

Managing mites in fruit crops requires that growers know when they have an unacceptable number of mites in the orchard for a given tree-stress condition as well as the number of predator beetles. However, the farmer must leave some mites alive in the orchard or the beetles will leave to find food elsewhere. In order to determine the number of mites in an orchard the farmer samples or scouts for mites and predators according to a research-based protocol.

Definition: Sampling is a statistical procedure that allows the estimation of population density by counting only a portion of the population in a very structured way. Counting only a tiny portion of the population saves time and provides acceptably accurate population estimates for decision-making purposes.

To help farmers decide when there are too many pests, the concept of the "Economic Injury Level (EIL)" is used. The EIL allows the farmer to compare the value of the damage the number of pests in the field might do to the crop with the cost of taking action against the pest. In other words, is the cost of taking action (e.g spray) more or less than the value of crop lost to the pest if no action is taken? The point where the cost of control equals the value of loss is called the EIL.

Defintion: Economic threshold of pest population density where the cost of control equals the value of the damage prevented if a control treatment is applied.

There is one more concept that is important. Given that we can calculate the EIL, by the time that the farmer determines that the pest population is getting to unacceptable levels and finds the time, equipment and help he/she needs to take action, the pest population has had a chance to exceed the EIL and eat into the farmer's profit. To account for this management 'lag' another measure, the Economic Threshold sometimes called Action Threshold, has been calculated to account for the farmer's reaction time.

Definition: Economic Threshold (ET) is the EIL minus some portion of the pest density that accounts for management lag of the farmer.

These thresholds are pre-calculated by researchers, so all the farmer has to do is take a proper sample of the pest to answer the question: Are we above or below the Economic Threshold for pest X?

To calculate Economic Threshold you must:

  • know how to identify the pest
  • know how to sample the crop environment to assess level of infestation
  • know stage of crop development & how that relates to severity of damage
  • know approximate economic threshold levels (available from Penn State Extension)
  • consider how action threshold may vary with stage of crop development, value of crop and cost of control.

Sampling for mites involves randomly selecting several trees in a given orchard. From each of these trees a number of random leaves are inspected with a hand lens. The number of mites is counted on each leaf. Averaging all of these samples will give and average 'mites per leaf' value for the orchard.

Sampling for the black ladybird is carried out by randomly selecting a number of trees from the orchard. The farmer walks slowly around the tree and, using a stop watch, counts all of the beetles he/she sees in 3 minutes. These counts are combined to give an average beetle count for the orchard.

Analyzing the sampling information is a two-step process. The first step is to determine if the mites exceed the economic threshold given the stress condition of the trees. This accomplished using the graph in Figure 1.

In figure 1 the vertical axis is the range of mites per leaf (from the mite sampling) that may be encountered. The horizontal axis is a range of 'crop loads' or the number of bushels of apples per acre a farmer expects to get from that orchard. The crop load is an important stress factor for the tree. The three plotted lines on the graph are three economic thresholds (ET). Each economic threshold relates to a particular time in the growing season.

Inspect the graph and note the trends:

As the growing season progresses, the economic thresholds rise. Older trees are more tolerant to mite feeding than younger trees so ET are low in the beginning of the season and high at the end.

As crop load increases, the economic threshold decreases. Apples and mites are both stress factors for the trees. When there are a lot of apples on the tree the farmer does not want as many mites.

The farmer selects the proper ET line on the graph based on date (time in the growing season), selects a point on the ET line that corresponds to the orchards crop load and determines if the mites he counted during the sampling exceed the ET. If mites are not over the ET then the farmer needs to take no further action and will return to the orchard in a week to sample again.

If mites are over the threshold then the second step of the analysis process is brought to bear-determining the predator/prey ratio. The predator/prey ration is a measure of the number of beetles in the orchard relative to the number of mites. Through research it has been determined that a predator/prey ratio of 2.5 or greater is needed for the beetles to suppress the mites to acceptable levels. In other words the number of beetles counted in three minutes should be equal to or greater than 2.5 times the number of mites per leaf. Even if the mites are over the ET, as long as the predator/prey ratio exceeds 2.5 the grower needs to take no further action, but should return in a week to reassess the situation.

If the predator/prey ratio is less that 2.5, the beetles will not be able to eat enough to suppress the mites. In this case the farmer should apply a pesticide that kills some of the mites to give beetles a chance to catch up.

It's obviously cheaper for the farmer if the beetles control the mites rather than spraying expensive pesticides. In addition, there is less environmental impact.

Since we cannot always go out to an apple orchard and sample in real life, this exercise simulates an orchard by having paper bag "trees" full of paper "leaves" with "mites" on them. Students will "sample" the leaves on the trees in the "orchard" and complete real data sheets just as a farmer would do in a real orchard. Then students compare the sample results with economic threshold values, determine tree stress and calculate predator/prey ratios to advise the grower what he or she should do about the mites, if anything.

Learning objectives:

  • Learn about the life cycles and habits of the European red mite and black ladybird beetle.
  • Learn about how trees grow and what stresses them.
  • Learn about Economic Injury Levels and Economic Threshold.
  • Learn about the profit concept as it applies to IPM.
  • Learn a pest population sampling technique
  • Compare sampling results to Economic Threshold to determine management action.
  • Learn how stage of crop development and other factors influences thresholds.

Materials needed:

  • Background information on pest & thresholds (see Penn State websites and publications)
  • Paper grocery bags
  • Construction paper (green and black)
  • Data sheet
  • Decision Graph (can be photocopied from this text).

Timeline: 45 minutes total

  • 20 minutes to explain concepts
  • 15 minutes to collect data
  • 10 minutes to discuss


Preparing materials:

  1. Each paper bag represents an apple tree. Five bags represent an orchard. The set-up information for five orchards (A, B, C, D, and E) follows this lesson. Several pest/crop/biocontrol scenarios can be represented by constructing several sets of paper bag orchards with varying values of mites, beetles and crop loads.
  2. On the side of the bag indicate the projected crop load for that 'orchard.' See set-up information for orchard crop loads.
  3. Cut the green construction paper into leaf shapes about 2 inches long (20 per bag in each orchard). Use a red magic marker to place red "dots" (mites) on the leaves. Number of mites per leaf should vary. Some leaves should not have mites. See set-up information for approximate numbers. Place 20 'leaves' into each bag in each orchard.
  4. Cut black construction paper into 1 inch circles. Glue the black circles on the outside of the bag (see set-up information for numbers). These circles represent black ladybird beetles counted on that 'tree' in three minutes.
  5. Arrange the bags on a table to form an orchard. Several orchards (each having characteristic mite and beetle populations) can be placed on other tables (or on the floor).

Data Collection:

  1. Each sample tree is approached by a scout (student). Ten random 'leaves' are withdrawn from the bag. The number of mites on each 'leaf' is counted and recorded on the data sheet.
  2. The number of 'beetles' is counted on the side of the bag and recorded on the data sheet.
  3. The expected crop load for that orchard is noted and recorded on the data sheet.


  1. Compute means (averages) for both mite samples and beetle samples.
  2. Based on time of season and expected crop load find the appropriate ET in Figure 1.
  3. Compare mean mite count obtain from sample with ET.
  4. If sample mean is equal to or greater than ET then farmer should assess beetle numbers. If the sample mean is less than ET then farmer does nothing and samples again next week.
  5. If indicated, compute the predator/prey ratio. If greater than 2.5, do nothing. If less than 2.5, apply miticide.


As you can see, there is not hard and fast answer for "how many pests are too many?" The answer is always 'It depends!' Discussions with students can center around the variables that determine real-world, on-farm decisions.

  1. Have each team describe their sampling results and ET comparisons.
  2. Ask about what actions the farmer should take given the sampling results.
  3. How should the farmers action change if; bushels per acre increases/decreases.
  4. What if the same sampling results were found in a younger/older crop?
  5. In addition to beetles and pesticides what other tactics can be used against mites in an IPM program?

Sample scenarios:

Crop load (bushels/acre)300500
DateJune 10July 20
Total black ladyird beetles753

Summary of data:

MiteAbove ThresholdAbove Threshold
Laybird beetleSufficientInsufficient
Recommended actionNothingApply pesticide

Paper Bag Orchard Set-up Information

Orchard A
treeCrop LoadMites/Leaf (20 leaves per tree)Black Ladybird Beetles
Tree 1250~7516
Tree 2250~8015
Tree 3250~8513
Tree 4250~7015
Tree 5250~8513
Orchard B
treeCrop LoadMites/Leaf (20 leaves per tree)Black Ladybird Beetles
Tree 1300~1035
Tree 2300~1032
Tree 3300~1031
Tree 4300~1532
Tree 5300~1031
Orchard C
treeCrop LoadMites/Leaf (20 leaves per tree)Black Ladybird Beetles
Tree 1450~ 817
Tree 2450~1210
Tree 3450~129
Tree 4450~2011
Tree 5450~169
Orchard D
treeCrop LoadMites/Leaf (20 leaves per tree)Black Ladybird Beetles
Tree 1200~7017
Tree 2200~7033
Tree 3200~7031
Tree 4200~7032
Tree 5200~7031
Orchard E
treeCrop LoadMites/Leaf (20 leaves per tree)Black Ladybird Beetles
Tree 1200~1133
Tree 2200~1146
Tree 3200~1140
Tree 4200~ 834
Tree 5200~1030

Graph: Action thresholds for mites in Pennsylvania