Agronomic Crop Insects

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

Alfalfa
Economic levels of alfalfa weevil larvae and pea aphids can both be found in alfalfa fields, especially in Kent and Sussex counties. When sampling for aphids and weevils, collect a minimum of 30 random stems throughout a field and place them top first in a white bucket. For aphids, you want to count the number present per plant as well as any that have dislodged from the stem into the bucket. 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. Although beneficial insects can help to crash aphid populations, the cooler temperatures have slowed their activity. As a general rule, you need one beneficial insect for every 50-100 aphids to help crash populations. For alfalfa weevil, you will also want to record the number of weevil larvae 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.

Field Corn
Black cutworm moth catches have significantly increased in a number of areas around the state, including Delmar, Leipsic and Selbyville (http://ag.udel.edu/extension/IPM/traps/currentbcwtraps.html). Moth catches of 9 to 15 moths per 7-day period have been associated with a moderate to high potential for cutworm outbreaks in field corn. Although pheromone trap catches can help determine when peak moth flight and egg laying occurs, they cannot predict the amount or magnitude of cutting that will occur. The presence of a major flight only means that the potential for an outbreak exists. Adverse weather, lack of adequate food for newly hatched larvae, predation, and disease can reduce larval populations. Scouting of seedling corn near the first cutting date is the best way to determine whether a problem exists. Even if a preventative treatment was used, all fields should be scouted at emergence for cutworm activity. As a general guideline, a treatment should considered in 1-2 leaf stage corn if you can find plants with 10% leaf feeding or 3% cut plants.

Small Grains
During the past week, we have received numerous calls regarding aphid management in barley and wheat. Based on research done in VA in past years, they found that small grains can tolerate a lot of feeding, especially lower in the canopy. As a general guideline, the treatment threshold for aphids in wheat and barley over a foot tall is 300 aphids per foot of row. 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, the cooler temperatures have slowed their activity. As a general rule, you need one beneficial insect for every 50-100 aphids to help crash populations.

We have also found low levels of cereal leaf beetle egg laying and the first small larvae in small grains in Sussex County. With the warm temperatures predicted for the end of this week, we could see an increase in larval activity by next week. The following information was taken from Dr. Ames Herbert’s fact sheet on cereal leaf beetle, which can be found at the following link: http://www.ext.vt.edu/pubs/entomology/444-350/444-350.html.

“Scout after peak egg laying and when up to 50% of eggs have hatched. If the population is mainly made up of eggs, then scouting should be at a later date when a minimum of 50% are small larvae. Samples should be taken at a minimum of 10 random sites in the interior of each field (avoid the edges). At each site, 10 tillers (stems) should be examined for eggs and larvae. This will result in 100 tillers (stems) per field being examined. Eggs may be on the leaves near the ground. Record the number of eggs and larvae counted at each sample site and calculate the total number of eggs + larvae found. Alternatively, stems can be examined at random while walking through the major portion of the field; again 100 stems per field should be examined. Scouting Frequency: Once egg laying has reached a peak, many fields will need only a single scouting for eggs and larvae. If the proportion of eggs in the sample is 50% or greater, then sample again in 5-7 days. Economic Threshold: 25 eggs and/or small larvae total per 100 tillers. This threshold is based on the number of eggs and small larvae present, rather than large larvae. Proper use allows fields at risk to be identified and treated in time to prevent significant yield loss.”

Very low levels of the first small grass sawfly and armyworm larvae have also been found in Sussex county. However, moth flights are behind compared to past years due to the cooler temperatures (especially night temperatures) over the past few weeks. If you treated a few weeks ago for aphids or winter grain mites, you will need to continue to scout for the “worm pests” since those sprays were applied too early to control armyworms and sawflies. In addition, if flag leaves have emerged already and you have typically applied a fungicide/insecticide application on wheat at flag leaf emergence, combination sprays applied this week and maybe even next week, may be too early to control armyworms and sawflies this year. In many years ¬†one application of an insecticide, timed when fungicide applications were needed, has been enough to control the complex of insects present; however, delayed moth catches means that peak moth laying and egg hatch will not occur until later this year. As a reminder, this combination spray has generally not been done before the last week in April in past years. Therefore, it will be important to sample fields that were treated early to be sure you do not miss an infestation of these “worm pests”.

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