Posts Tagged ‘alfalfa weevil’

Agronomic Crop Insects – April 1, 2011

Friday, April 1st, 2011

Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu

Alfalfa
Alfalfa Weevil:
We are starting to see the first hatch of alfalfa weevil eggs. As soon as the weather begins to warm up, you should begin to sample for larvae on a weekly basis. Look for small larvae feeding in the tips of plants producing a round, pinhole type of feeding. Once you detect tip feeding, a full field sample should be taken. The most accurate way to time an application is to sample stems and determine the number of weevils per stem. A minimum of 30 stems should be collected per field and placed top first in a bucket to dislodge larvae from the tips. Then count the number of weevils per stem. The following thresholds, based on the height of the alfalfa, should be used as a guideline when making a treatment decision: up to 11 inches tall – 0.7 per stem; 12 inches tall – 1.0 per stem; 13 – 15 inches tall – 1.5 per stem; 16 inches tall – 2.0 per stem; and 17 – 18 inches tall – 2.5 per stem. Numerous pyrethroids are now labeled for alfalfa weevil including Baythroid XL, Mustang MAX, Proaxis, Warrior II and numerous generic pyrethroids. Imidan, Lorsban, Lannate and Steward are also labeled for alfalfa weevil control. Be sure to check all labels for rates, restrictions and days to harvest before application. The following is a link to our recently updated fact sheet including pictures of life stages and damage.  http://ag.udel.edu/extension/IPM/ExtensionFactSheets/AlphalfaWeevilIPM-1.pdf.

Timothy
Cereal Rust Mites:
Since spring green-up is underway, be sure to sample fields for cereal rust mite activity. Mites can be found in fields at this time. These mites are very small, so the use of a 20x-magnifying lens may be helpful. If rust mites become a problem, Sevin XLR Plus is still the only labeled, effective material: http://www.cdms.net/LDat/ld332013.pdf . Be sure to read the label for information on the number of applications per season as well as the days to harvest. For effective rust mite control, the use of the higher labeled rate and at least 25 gal/A of carrier to get good coverage of leaf surfaces generally results in better control. The following is a link to new fact sheet including pictures of mites and damage: http://ag.udel.edu/extension/IPM/ExtensionFactSheets/CerealRustMiteIPM-9.pdf.

Wheat
Cereal Leaf Beetle:
It is time to begin sampling fields for cereal leaf beetle activity. We are starting to find evidence of adult feeding, so fields should now be scouted for egg masses. The threshold for cereal leaf beetle includes sampling for eggs, especially in high management wheat fields or in fields with historical problems. The eggs are elliptical, about 1/32 inch long, yellow in color when first laid, changing to a burnt orange prior to hatching. The following is a link to our recently updated fact sheet including pictures of life stages and damage: http://ag.udel.edu/extension/IPM/ExtensionFactSheets/CerealLeafBeetleFactSheetIPM-5.pdf.

Generally, eggs are laid singly or in small scattered groups (end-to-end) on the upper leaf surface and parallel to the leaf veins. Cereal leaf beetle larvae are brown to black, range in size from 1/32 to ¼ inch long, and eat streaks of tissue from the upper leaf surface. Since cereal leaf beetle populations are often unevenly distributed within the field, it is important to carefully sample fields so that you do not over or under estimate a potential problem. Eggs and small larvae should be sampled by examining 10 tillers from 10 evenly spaced locations in the field while avoiding field edges. This will result in 100 tillers (stems) per field being examined. Eggs and larvae may be found on leaves near the ground so careful examination is critical. You should also check stems at random while walking through a major portion of the field and sampling 100 stems. The treatment threshold is 25 or more eggs and/or small larvae per 100 tillers. If you are using this threshold, it is important that you wait until at least 50% are in the larval stage (i.e. after 50% egg hatch).

Winter Grain Mites: With the recent cooler weather, consultants are starting to report an increase in winter grain mite populations, especially in no-till wheat planted into corn stubble. Temperature and moisture are the most important factors influencing mite development and abundance. Cool, rather than warm, temperatures favor their development. Egg laying is heaviest between 50° and 60°F and the optimum conditions for hatching are between 44° and 55°F. Mite activity in the spring drops rapidly and the eggs fail to hatch when the daily temperature exceeds 75°F. The larvae as well as the adults feed higher up on the plants at night or on cloudy days. Heavily infested fields appear grayish or silvery, a result of the removal of plant chlorophyll by mite feeding. When high infestations feed on the plants for several days, the tips of the leaves exhibit a scorched appearance and then turn brown, and the entire plant may die. These mites do not cause the yellowing characteristic of spider mite feeding. Many of the infested plants do not die, but become stunted and produce little forage or grain; damage on young plants, however, is more severe than on large, healthy ones. Damage may also be greater in plants stressed by nutrient deficiencies or drought conditions. There are two types of damage to the small grain: (a) reduced amount of forage throughout the winter and (b) reduced yields of grain in the spring and summer. The following is a link to our recently updated fact sheet including pictures of mites: http://ag.udel.edu/extension/IPM/ExtensionFactSheets/WinterGrainMitesIPM-8.pdf.

The most effective scouting method is to use a 10x hand lens, checking both plant foliage and crop residue on the soil surface for the presence of immature and adult mites. A sweep net may also be effective in determining if mites are present. The best time to scout is early in the morning and at dusk on calm days because the mites will seek refuge during the day in the top 4 or 5 inches of the soil profile to avoid the sunlight. On cool, overcast days, they may be observed actively feeding on plant foliage throughout the day.

No economic thresholds have been developed for WGM in small grain fields. However, as a general rule of thumb, if plants exhibit symptoms of damage, weather conditions are favorable and several mites per plant are found, a chemical control may be necessary to prevent or reduce yield loss. If populations are small and the plants show no feeding injury or if populations and damage symptoms are isolated, the field should be scouted more frequently to insure yield losses are kept at a minimum.

Agronomic Crop Insects

Thursday, April 22nd, 2010

Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu

Alfalfa
Continue to scout fields for both alfalfa weevil and pea aphids. Economic levels of both can be found in alfalfa fields at this time. As a general guideline, you should consider a treatment in alfalfa less than 10 inches tall if you find 40-50 aphids per stem. The treatment threshold for alfalfa 10 inches or taller in height is 75- 100 per stem. Although beneficial insects can help to crash aphid populations, cooler temperatures will slow their activity. As a general rule, you need one beneficial insect per every 50-100 aphids to help crash populations. As soon as temperatures increase, we will start to see a significant increase in feeding damage from alfalfa weevil. As alfalfa approaches harvest, the decision to cut instead of treat may be considered. However, this option should only be used if you plan to cut shortly after you find an economic threshold level since damage can occur quickly. Cutting should only be considered as a management option if you can cut within 3- 5 days of finding an economic level. As you get close to harvest, be sure to check labels carefully for time between application and harvest.

Field Corn
As soon plants emerge, be sure to check for cutworm feeding, even if an at planting insecticide was used for cutworm control. A combination of wet conditions and the extended heavy snow cover has resulted in a higher level of slugs being found under residue in no-till fields. Although no true thresholds are available, levels of 5 or more slugs per square foot have indicated the potential for a problem from slugs.

Small Grains
Although aphid population remain low, a combination of cool temperatures followed by a quick increase in temperatures can result in a quick increase in aphid populations. In research done in VA in past years they found that small grains can tolerate a lot of feeding in the lower canopy. Since we are past the time of barley yellow dwarf transmission (fall transmission is the most important), the next important time to consider aphid management in small grains is at grain head emergence. Since aphids feeding in the heads of small grains can result in a loss in test weight, it is important to look for aphids as soon as the grain heads emerge. As a general guideline, a treatment should be considered if you find 20 aphids per head and beneficial insect activity is low. Although beneficial insects can help to crash aphid populations, cooler temperatures will slow their activity. As a general rule, you need one beneficial insect per every 50-100 aphids to help crash populations. Since barley heads have emerged, be sure to watch for the movement of aphids into grain heads. In many cases, beneficial activity is still not high enough to take care of populations moving from the lower canopy of the plants into the grain heads.

Once grain heads have emerged, you should also begin sampling small grains for grass sawfly and armyworm larvae. Remember, armyworm larvae are nocturnal so look for larvae at the base of the plants during the day. As a general guideline, a treatment should be considered if you find one armyworm per foot of row for barley and 1-2 per foot of row for wheat. The first small sawflies have been found in wheat and barley in Kent and Sussex counties. Since sawflies feed on the plants during the day, small sawfly larvae can often be detected early using a sweep net. However, there is no threshold for sweep net samples. Once sawfly larvae are detected, sample for larvae in 5 foot of row innerspace in 5-10 locations in a field to make a treatment decision. You will need to shake the plants to dislodge sawfly larvae that feed on the plants during the day. As a guideline, a treatment should be applied when you find 2 larvae per 5 foot of row innerspace or 0.4 larvae per foot of row. If armyworms and sawflies are present in the same field, the threshold for each should be reduced by one-half.

Agronomic Crop Insects

Thursday, April 1st, 2010

Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu

Alfalfa
Now that the weather has finally started to warm up, you should begin to sample for alfalfa weevil on a weekly basis. Look for small larvae feeding in the tips of plants producing a round, pinhole type of feeding. Once you detect tip feeding, a full field sample should be taken. In general, no treatment should be needed before you observe 50% of the tips with feeding damage. The most accurate way to time an application and try to avoid multiple insecticide applications is to sample stems and determine the number of weevils per stem. A minimum of 30 stems should be collected per field and placed top first in a bucket to dislodge larvae from the tips. Then count the number of weevils per stem. The following thresholds, based on the height of the alfalfa, should be used as a guideline when making a treatment decision: up to 11 inches tall – 0.7 per stem; 12 inches tall – 1.0 per stem; 13 – 15 inches tall – 1.5 per stem; 16 inches tall – 2.0 per stem; and 17 – 18 inches tall – 2.5 per stem. Numerous pyrethroids are now labeled for alfalfa weevil including Baythroid XL, Mustang MAX, Proaxis, Warrior II and numerous generic pyrethroids. Imidan, Lorsban, Lannate and Steward are also labeled for alfalfa weevil control. Be sure to check all labels for rates, restrictions and days to harvest before application. NOTE: The use of Furadan on alfalfa (as well as all other crops) has been cancelled and existing stocks CAN NOT be used.

Field Corn
In addition to black cutworm (which is generally a pest of later planted corn), we can also have a number of other cutworm species present in corn fields at planting time, including the dingy cutworm, claybacked cutworm and variegated cutworm. Information from the Midwest indicates that the claybacked cutworms can cause economic loss in corn. They overwinter as half-grown larvae in the soil so they can get a “jump” on black cutworms when it comes to cutting each spring. Since they are larger in size earlier in the spring, this species can damage very young corn plants. So, scouting fields at plant emergence is important, even if at-planting materials were used, to catch any potential problems. Just a reminder, if you plan to tank-mix an insecticide with an herbicide for black cutworm control, it should be done at, or immediately following planting.

During the winter meeting season, there was a lot of discussion regarding changes in refuge requirements for corn expressing Bt traits for the upcoming 2010 growing season. As of this date, the refuge in a bag strategy has not been approved for any transgenic corn product. However, there has been one significant change in refuge requirements for 2010 involving SmartStax hybrids. Because these hybrids contain 3 different genes for Lepidopteran control and 3 different genes for rootworm control, the EPA has approved a reduction of the standard 20% refuge to 5% refuge. In the last few weeks, I also received new information from Monsanto regarding placement of the refuge for SmartStax hybrids that applies to our area:

“The common refuge can be within or adjacent to the Genuity™ SmartStax™ field. If adjacent, it can be separated by a road, path, ditch , etc., but not by another field. Monsanto recommends planting the corn refuge for Genuity™SmartStax™ as an in-field or adjacent refuge as explained in this IRM/Grower Guide. However, if corn rootworms are not significant within a region, the common refuge may also be planted as a separate block that is within 1/2 mile of the Genuity™ SmartStax™field. This additional option to plant the refuge as a block within 1/2 mile is only available to farmers in the following States: AK, AL, AR, AZ, CA, CT, DE, FL, GA, HI, ID, LA, MA, MD, ME, MS, MT, NC, NH, NJ, NM, NV, NY, OR, PA, RI, TN, SC, UT, VA, VT, WA, WV, WY.”  Monsanto’s 2010/IRM Insect Resistance Management Grower Guide will be posted to their webpage in the near future. Also, be sure to contact your seed dealers for a full description of refuge requirements for 2010 for all hybrids that you plan to plant.

Timothy
Since spring green up is underway, be sure to sample fields for cereal rust mite activity. Mites can be found in fields at this time. These mites are very small, so the use of a 20x-magnifying lens may be necessary. If rust mites become a problem, Sevin XLR Plus (which had a 24(c) label on timothy for cereal rust mite management) now has a full section 3 federal label which includes pastures and grasses grown for hay and seed. The following is a link to the new label: http://www.cdms.net/LDat/ld332012.pdf. Be sure to read the label since there is new information on the number of applications per season as well as the days to harvest. For effective rust mite control with Sevin, the use of the higher labeled rate and at least 25 gal/acre of carrier to get good coverage of leaf surfaces generally results in better control.

Wheat
It is time to begin sampling fields for cereal leaf beetle activity. We are starting to find evidence of adult feeding, so fields should now be scouted for the presence of egg masses. The threshold for cereal leaf beetle has been adjusted to include sampling for eggs, especially in high management wheat fields or areas where problems were experienced the previous year. The eggs are elliptical, about 1/32 inch long, orange to yellow in color when first laid, changing to a burnt orange prior to hatching. Check our website for pictures of cereal leaf beetle adults, larvae and eggs: http://www.udel.edu/IPM/facts/clbpictures.htm.

Generally, eggs are laid singly or in small scattered groups (end-to-end) on the upper leaf surface and parallel to the leaf veins. Cereal leaf beetle larvae are brown to black, range in size from 1/32 to 1/4 inch long, and eat streaks of tissue from the upper leaf surface. Since cereal leaf beetle populations are often unevenly distributed within the field, it is important to carefully sample fields so that you do not over or under estimate a potential problem. Eggs and small larvae should be sampled by examining 10 tillers from 10 evenly spaced locations in the field while avoiding field edges. This will result in 100 tillers (stems) per field being examined. Eggs and larvae may be found on leaves near the ground so careful examination is critical. You should also check stems at random while walking through a major portion of the field and sampling 100 stems. The treatment threshold is 25 or more eggs and/or small larvae per 100 tillers. If you are using this threshold, it is important that you wait until at least 50% are in the larval stage (i.e. after 50% egg hatch).

Agronomic Crop Insects

Friday, April 3rd, 2009

Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu

Alfalfa
If you have not started to sample for alfalfa weevil, be sure to begin sampling fields on a weekly basis. Larvae are present in fields in Kent and Sussex counties. Look for small larvae feeding in the tips of plants producing a round, pinhole type of feeding. Once you detect tip feeding, a full field sample should be taken. In general, no treatment should be needed before you observe 50 percent of the tips with feeding damage. The most accurate way to time an application and try to avoid multiple insecticide applications is to sample stems and determine the number of weevils per stem. A minimum of 30 stems should be collected per field and placed top first in a bucket to dislodge larvae from the tips. Then count the number of weevils per stem. The following thresholds, based on the height of the alfalfa, should be used as a guideline when making a treatment decision: up to 11 inches tall – 0.7 per stem; 12 inches tall – 1.0 per stem; 13 – 15 inches tall – 1.5 per stem; 16 inches tall – 2.0 per stem and 17 – 18 inches tall – 2.5 per stem. Numerous pyrethroids are now labeled for alfalfa weevil including Baythroid XL, Mustang MAX, Proaxis, Warrior II and numerous generic pyrethroids. Imidan, Lorsban, Lannate and Steward are also labeled for alfalfa weevil control. Be sure to check all labels for rates, restrictions and days to harvest before application. NOTE: The use of Furadan on alfalfa has been cancelled (as well as 21 other crop uses). According to EPA, “existing stocks of the canceled products may be used until they are depleted, or until the effective date for revocation of the associated tolerances.”

Field Corn
In addition to black cutworm (which is generally a pest of later planted corn), we can also have a number of other cutworm species present in corn fields at planting time, including the dingy cutworm, claybacked cutworm and variegated cutworm. Information from the Midwest indicates that the claybacked cutworms can cause economic loss in corn. They overwinter as half-grown larvae in the soil so they can get a “jump” on black cutworms when it comes to cutting each spring. Since they are larger in size earlier in the spring, this species can damage very young corn plants. So, scouting fields at plant emergence is important, even if at-planting materials were used, to catch any potential problems. Just a reminder, if you plan to tank-mix an insecticide with an herbicide for black cutworm control, it should be done at, or immediately following planting. Insecticides combined with early burn-down applications, 2-3 weeks before planting, have not provided effective control.

Timothy
Be sure to sample fields for cereal rust mite activity. Mites are active in fields at this time. As soon as fields green up, you should begin checking for cereal rust mites and the early signs of infested leaves, especially in fields with problems in past years. These mites are microscopic, so the use of a 20x-magnifying lens is necessary. If rust mites become a problem, Sevin XLR Plus still has a 24(c) label on timothy for rust mite management in DE. The following is a link to the 24(c) label for Delaware: http://www.cdms.net/ldat/ld332028.pdf. You must have this label in your possession at the time of application.

Wheat
Be sure to begin sampling fields for cereal leaf beetle activity. We are starting to find evidence of adult feeding, so fields should be scouted early for the presence of egg masses. The threshold for cereal leaf beetle has been adjusted to include sampling for eggs, especially in high management wheat fields or areas where problems were experienced the previous year. The eggs are elliptical, about 1/32 inch long, orange to yellow in color when first laid, changing to a burnt orange prior to hatching. Check our website for pictures of cereal leaf beetle adults, larvae and eggs: http://www.udel.edu/IPM/facts/clbpictures.htm.

Generally, eggs are laid singly or in small scattered groups (end-to-end) on the upper leaf surface and parallel to the leaf veins. Cereal leaf beetle larvae are brown to black, range in size from 1/32 to 1/4 inch long, and eat streaks of tissue from the upper leaf surface. Since cereal leaf beetle populations are often unevenly distributed within the field, it is important to carefully sample fields so that you do not over or under estimate a potential problem. Eggs and small larvae should be sampled by examining 10 tillers from 10 evenly spaced locations in the field while avoiding field edges. This will result in 100 tillers (stems) per field being examined. Eggs and larvae may be found on leaves near the ground so careful examination is critical. You should also check stems at random while walking through a major portion of the field and sampling 100 stems. The treatment threshold is 25 or more eggs and/or small larvae per 100 tillers. If you are using this threshold, it is important that you wait until at least 50% are in the larval stage (i.e. after 50% egg hatch).

Agronomic Crop Insects

Friday, April 11th, 2008

Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu

Alfalfa
If you have not started to sample for alfalfa weevil, be sure to begin sampling fields on a weekly basis. Look for small larvae feeding in the tips of plants producing a round, pinhole type of feeding. Once you detect tip feeding, a full field sample should be taken. In general, no treatment should be needed before you observe 50 percent of the tips with feeding damage. A more accurate way to time an application and try to avoid multiple insecticide applications would be to sample stems and determine the number of weevils per stem. A minimum of 30 stems should be collected per field, placed top first in a bucket to dislodge larvae from the tips and then count the number of weevils per stem. The following thresholds, based on the height of the alfalfa, should be used as a guideline when making a treatment decision: up to 11 inches tall – 0.7 per stem; 12 inches tall – 1.0 per stem; 13 – 15 inches tall – 1.5 per stem; 16 inches tall – 2.0 per stem and 17 – 18 inches tall – 2.5 per stem. Numerous pyrethroids are now labeled for alfalfa weevil including Baythroid, Mustang MAX, permethrin, Proaxis and Warrior. Furadan, Imidan, Lorsban, Lannate and Steward are also labeled for alfalfa weevil control.

Field Corn
In general, black cutworm trap catches remain low. However, we have started to see an increase in catches this past week. Although no precise numbers are available, moth catches of 9 to 15 moths per 7-day period have been associated with a moderate to high potential for black cutworm outbreaks in field corn. Larvae should be large enough to begin cutting when about 300 base-50 degree-days have accumulated since peak moth activity and egg laying. Pheromone trap catches can help determine when peak moth flight and egg laying occurs; however, they cannot predict the amount or magnitude of cutting that will occur.

We can also have a number of other cutworm species present in corn fields at planting time including the dingy cutworm, claybacked cutworm and variegated cutworm. Reports from consultants indicate that they have found higher numbers of cutworms while doing soil sampling for grubs earlier this spring. Information from the Midwest indicates that the claybacked cutworms can cause economic loss in corn. They overwinter as half-grown larvae in the soil so they can get a “jump” on black cutworms when it comes to cutting each spring. Since they are larger in size earlier in the spring, this species can damage very young corn plants. So, scouting fields at plant emergence is important, even if at planting materials were used, to catch any potential problems.

Just a reminder, if you plan to tank-mix an insecticide with an herbicide for black cutworm control, it should be done at, or immediately following planting. Insecticides combined with early burn-down applications, 2-3 weeks before planting, have not provided effective control. For the most recent pheromone trap catches, be sure to check trap catches posted weekly on the University of Delaware IPM website at (http://ag.udel.edu/extension/IPM/traps/currentbcwtraps.html)

Timothy
Be sure to sample fields for cereal rust mite activity. As soon as fields green up, you should begin checking for cereal rust mites and the early signs of infested leaves, especially in fields with problems in past years. These mites are microscopic, so the use of a 20x-magnifying lens is necessary. If rust mites become a problem, Sevin XLR Plus still has a 24(c) label on timothy for rust mite management. The following is a link to the 24(c) label for Delaware. (http://www.cdms.net/ldat/ld332028.pdf). You must have this label in your possession at the time of application.

Wheat
In addition to sampling for aphids, be sure to begin sampling fields for cereal leaf beetle activity. We can find evidence of adult feeding, so fields should be scouted early for the presence of egg masses. In recent years, the threshold for cereal leaf beetle has been adjusted to include sampling for eggs, especially in high management wheat fields or areas where problems were experienced the previous year. The eggs are elliptical, about 1/32 inch long, orange to yellow in color when first laid, changing to a burnt orange prior to hatching. Check our website for pictures of cereal leaf beetle adults, larvae and eggs: http://www.udel.edu/IPM/facts/clbpictures.htm

Generally, eggs are laid singly or in small scattered groups (end-to-end) on the upper leaf surface and parallel to the leaf veins. Cereal leaf beetle larvae are brown to black, range in size from 1/32 to 1/4 inch long, and eat streaks of tissue from the upper leaf surface. Since cereal leaf beetle populations are often unevenly distributed within the field, it is important to carefully sample fields so that you do not over or under estimate a potential problem. Eggs and small larvae should be sampled by examining 10 tillers from 10 evenly spaced locations in the field while avoiding field edges. This will result in 100 tillers (stems) per field being examined. Eggs and larvae may be found on leaves near the ground so careful examination is critical. You should also check stems at random while walking through a major portion of the field and sampling 100 stems. The treatment threshold is 25 or more eggs and/or small larvae per 100 tillers. If you are using this threshold, it is important that you wait until at least 50% are in the larval stage (i.e. after 50% egg hatch).

Scout Alfalfa for Alfalfa Weevil and Pea Aphids

Friday, March 28th, 2008

Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu

Be sure to sample alfalfa fields for small alfalfa weevil larvae feeding in the tips of plants. Early damage will appear as a round, pinhole type of feeding. Once you detect tip feeding, a full field sample should be taken. You will want to avoid treating fields too early since it may result in multiple applications. Also, be sure that you do not confuse clover leaf and alfalfa weevil larvae. Cloverleaf weevils are generally larger at this time of year and have a distinct white stripe lined with red down the middle of their backs. Although cloverleaf weevils can cause damage during cool, dry springs, controls are generally not needed for cloverleaf weevils. For pictures of cloverleaf weevil and alfalfa weevil, please refer to the following links:

http://www.ipm.iastate.edu/ipm/icm/1997/4-21-1997/icloverweevil.html
http://www.ent.iastate.edu/imagegal/coleoptera/curculionidae/0212.47alfalfalarva6in.html

You will also want to sample fields for pea aphids. Heavily infested plants may turn yellow and wilt. Pea aphids prefer cool, dry conditions and can be a problem in both the first cutting and during spring seedling establishment. This species tends to congregate on the tips of alfalfa plants where they feed on young, succulent developing shoots. To sample for aphids, clip alfalfa stems at the base of the plant and record the number present per plant. You may want to examine plants over a white bucket to collect any aphids that are dislodged from the plants. In seedling stage alfalfa, a treatment should be considered if you find 5 aphids per stem. As a general guideline, you should consider a treatment in alfalfa less than 10 inches tall if you find 40-50 aphids per stem. The treatment threshold for alfalfa 10 inches or taller in height is 75-100 per stem.