Alfalfa is an important forage crop to the Northeast. As a deep-rooted perennial, alfalfa is very beneficial in farmers' crop rotations. A legume, alfalfa can "fix" nitrogen and thus is an important protein source for dairy cattle. Depending on the year and location, alfalfa can be cut three or more times per year.
The potato leafhopper is the most destructive insect of alfalfa in Pennsylvania and the northeastern U.S. It causes yield losses, reduces alfalfa quality (especially protein content), and contributes to reduced longevity of stands. In new seedings, the pest can cause serious stand losses and weak plants.
The insect does not over winter in Pennsylvania, but in states along the Gulf of Mexico and up the eastern seaboard into southern Maryland. For this reason it must migrate into the state on spring storm fronts that originate in the Gulf of Mexico or along the eastern seaboard. The insect typically reaches Pennsylvania between May 1 and June 15. The time the pest arrives in the state and the intensity of pest pressure on the crop is related to the timing and frequency of these storm fronts. Because the pest tends not to arrive in most years until late May, the first cutting of alfalfa is seldom injured by the pest's feeding. Potato leafhopper is primarily a pest of the second and third cuttings of alfalfa in Pennsylvania.
The pest injures alfalfa plants by inserting its beak-like mouthparts into the cells that surround the phloem tissue and sucking out the cell contents. This causes the cells to swell around the phloem, pinching it shut and preventing carbohydrate movement in the plant. Injured plants are stunted and have yellow "V" shaped areas at the tips of leaves ("hopperburn"). In severely injured fields, plants may drop their leaves. Once plants are stunted by leafhopper they will not grow until after the alfalfa is cut.
Although the potato leafhopper infestations can cause severe injury to alfalfa, only a percentage of fields need to be treated each year. Because of weather patterns and the population growth characteristics of the pest, very few fields have worrisome infestations in some years. Other years, these factors lead to severe infestations. This variation in threat by the pest makes it advantageous for farmers to scout their fields and determine the need for control. This exercise is designed to teach potato leafhopper scouting methods and how to determine if control is needed.
In most crops, and most seasons, pest insect species that feed on the crop will be present at some point in the plant life cycle. However, just because the pest is present does not necessarily mean that the farmer needs to take action against the pest. How does the farmer know when the number of pests in his/her crop is too many? Is this number the same every year in all fields?
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.
Economic Injury Level (EIL) is the 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.
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.
To learn how this works, we will use alfalfa as the crop and potato leafhopper (PLH) as the pest. In actual practice, from May till final alfalfa harvest in the fall, PLH are 'sampled' using sweep nets.
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 sample, the farmer walks across the alfalfa field sweeping the net in front of him/her, skimming the tops of the plants to catch PLH. The number of sweeps and walking direction is predetermined by the sampling protocol. The farmer opens the sweep net and counts the number of PLH in it. The farmer compares the number of PLH caught with the economic threshold value in a decision table (Table 1). If the number of PLH in the net exceeds the Economic Threshold value, the farmer needs to take action against the pest. If the number in the net does not exceed the Economic Threshold, then the farmer does not need to take any action. This procedure is repeated periodically from the time PLH arrives from their hibernating area in the southern U.S. (usually in May) until the final harvest.
The decision tables containing the Economic Thresholds (Table 1) allow a decision to be made under different conditions including various plant heights, prices of alfalfa hay per ton and costs of an insecticide application per acre. These conditions relate measures of crop value and control costs as they affect the profit derived from the alfalfa field. In general, farmers try to maximize profit.
Profit equals income minus expenses. Therefore, profit can be increased either by increasing the income, decreasing the expenses, or both.
Inspect the decision tables and observe the trends in Economic Threshold:
As plant height increases the Economic Threshold increases. Younger plants are much more sensitive to insect damage. Damage to a young plant will significantly decrease the value of the plant (yield and quality) and reduce the farmer's income from that alfalfa field. Older plants can tolerate more insects and will not lose as much value so older plants (=taller) have higher Economic Threshold. In other words the farmer can let PLH populations get to higher levels in older plants before he/she has to control them. In addition, the most cost-effective action against the pest may be early harvest if crop maturity is "close enough".
As the price of alfalfa hay increases the Economic Threshold decreases. A given amount of damage by PLH to high priced alfalfa will decrease the value of the crop more than that same amount of damage to low priced alfalfa. The price of the hay is determined by the marketplace, so the farmer will have to guess what the price of alfalfa might be at the time he/she wants to sell it.
As the cost of control (insecticide application) increases the Economic Threshold increases. In terms of profit, an expensive control cost can result in lower profit unless the value of the crop is also high to compensate for it. If the alfalfa price does not justify the high control cost then Economic Threshold is allowed to rise (allowing more PLH to exist in the field before action must be taken).
Table 1. Potato Leafhopper Economic Threshold Chart for Established Stands of Alfalfa
|Crop||Cost of an Insecticide Application Per Acre|
|Height Category 1 - 0 to 4 inches|
|Height Category 11 - 5 to 8 inches|
|Height Category 111 - 9 to 12 inches|
To learn how this works, we will 'sample' an 'alfalfa field' and decide if we need to take action against PLH. Ideally, this exercise should be done in an alfalfa field in the mid-summer, but since most public schools do not meet in the summer, and not everyone has access to an alfalfa field, we are providing a classroom alternative-The Paper Bag Alfalfa Field!
1. Learn migration and life cycle patterns of a key alfalfa pest, the potato leafhopper.
2, Learn about Economic Injury Levels and Economic Threshold.
3. Learn about the profit concept as it applies to IPM.
2. Learn a pest population sampling technique
3. Compare sampling results to Economic Threshold to determine management action.
4. Learn how stage of crop development and other factors influences thresholds.
- Background information on pest & thresholds (see Penn State websites and publications)
- Paper grocery bags
- Construction paper (two colors)
- Data sheet
- Decision tables (can be photocopied from this text).
Timeline: 45 minutes total
- 20 minutes to explain concepts
- 15 minutes to collect data
- 10 minutes to discuss
- The paper or plastic bag represents the alfalfa field. Each bag (field) will be sampled, so enough bags are needed to present several pest/crop scenarios. We recommend at least three. If desired, the bags can be decorated by drawing an alfalfa plant on the side.
- On the side of the bag indicate the projected price per ton that is expected for the alfalfa in that 'field.' Also indicate the cost of an insecticide spray, if one should be needed, for that 'field.' (See "How to Create Alfalfa Field Paper Bags" at the end of this section.)
- Cut one of the colors of construction paper into 2 inch squares. On each square write a number representing the number of PLH found in each sweep sample. Scenarios should illustrate the different situations that the farmer may encounter. For instance, if one scenario is meant to demonstrate that there are not enough PLH to justify taking action, then the numbers written on the squares should tend to be low. (See "How to Create Alfalfa Field Paper Bags" at the end of this section for suggested numbers.)
- Cut the other color of construction paper into 2 inch diameter circles. On each circle write a number representing plant heights found in that field. (See "How to Create Alfalfa Field Paper Bags" at the end of this section for suggested numbers.)
- Place 20 numbered, paper squares and 20 numbered paper circles into each bag.
(See "Potato Leafhopper Worksheet" at the end of this section)
Simulating PLH sampling
- Form teams of two. One team member will do the sampling, the other will record data.
- The sampler reaches into the bag, pulls out one numbered paper square and reads the number to the recorder who records the number on the PLH count portions of the data sheet. Repeat 5 times.
- Similarly, the sampler retrieves a number circle from the bag and reads the number to the recorder who records the number in plant height column of the data sheet.
- Compute means (averages) for both PLH samples and plant height samples.
- Based on average plant height, hay price and control cost find the appropriate ET in Table 1.
- Compare mean PLH count obtain from sample with ET.
- If sample mean is equal to or greater than ET then farmer should apply an insecticide. If the sample mean is less than ET then farmer does nothing and samples again next week.
As you can see, there is no 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.
- Have each team describe their sampling results and ET comparisons.
- Ask about what actions the farmer should take given the sampling results.
- How should the farmers action change if; alfalfa hay price rises/falls, insecticide prices increases/decreases?
- What if the same sampling results were found in a younger/older crop?
- In addition to pesticides what other tactics can be used against PLH in an IPM program?
Developed by Ed Rajotte, Penn State