Emily Zobel, Senior Agriculture Agent Associate | ezobel@umd.edu University of Maryland Extension, Dorchester County
Alfalfa
Alfalfa weevils emerge and lay eggs in alfalfa stems in Mid-April. The larvae are yellowish-green with blackheads. The easiest way to scout for this pest is to randomly collect 30 stems from the field and shake them into a bucket. The economic threshold for alfalfa weevil is determined based on the height of plants, the value of the forage, and the cost of insecticidal treatment. A threshold chart can be found on Penn State Extension website https://extension.psu.edu/alfalfa-weevil (Figure 1).
Figure 1. Economic threshold for Alfalfa Weevil. Source: Penn State University
Small Grain
Cereal leaf beetle adults become active in Maryland around mid-April. Adults will lay eggs and larvae will start feeding around the end of the month. Scouting should be done away from the field edge since they tend to clump in fields and near edges. Check tillers of 50-60 randomly selected plants per field and count the number of eggs and larvae. The economic threshold is reached when there is an average of one or more larvae on 25% of tillers. The populations can be spotty, so checking individual fields is often necessary.
Several species of aphids will start to appear in small grains and other spring crops in April, depending on the weather. The most common species in our area are English grain aphid, bird cherry-oat aphid, corn leaf aphid, and the greenbug. Springtime feeding damage can cause discoloration on the leaves and shriveled heads. To scout for aphids, examine one linear row-foot at ten sites within the field. The economic threshold for aphids in wheat in pre-heading stages varies based on the aphid species present. Still, the general rule is treatment is recommended if there is an average of 150 aphids per linear foot of row, with no natural enemies present. For information about species identification and thresholds, check out the “Early Aphid Occurrences: a Possible Result of Warmer Winter Temperatures” article on the Maryland Agronomy News Blog, or contact your local Extension agent.
Darsy Smith, Graduate Student & Dr. William Lamp, Professor University of Maryland, Department of Entomology
Figure 1. Yellow appearance of the alfalfa field that lead to the discovery of the cowpea aphid outbreak at BARC. Photo courtesy of Russell Griffith.
An unexpected outbreak of cowpea aphid, Aphid craccivora Koch, in Maryland was discovered last month by Terry Patton, who was contacted by Russell Griffith, tractor operator leader at Beltsville Agricultural Research Center (BARC), because of the yellow appearance of an alfalfa field (Figure 1) and the infestation of dark aphids (Figure 4). Since the 1990s, infestations of the cowpea aphid have been observed in Maryland alfalfa, but this is an unusually large outbreak. Stay alert to this emerging pest and learn how to identify it since it has a wide range of hosts and may damage crops.
Figure 2. Adult cowpea aphid. Note the cornicle (yellow arrow) is dark and long and the abdomen (red arrow) is distinctive dark and shiny. Photo courtesy of influentialpoints.comFigure 3. Adult and nymph cowpea aphids. Nymph color is opaque and varies (yellow arrow) from brown to gray while the adult (red arrow) has the distinctive dark, shiny abdomen. Photo courtesy of Andrew Jensen, https://aphidtrek.org/
Cowpea aphid identification and injury. Cowpea aphid is not generally an economic pest in alfalfa but learning how to identify the aphid and its injury can help you prevent losses. Cowpea aphid is easily differentiated from other aphids in alfalfa because its dark coloration, with the abdomen of the adults much darker and shinier than the rest of its body (Figure 2). In addition, the cornicle (or siphuncule) is dark and long (Figure 2). The nymph is less shiny (opaque) and varies from brown to gray (Figure 3). The legs and antennae of both adults and nymphs are pale with dark tips (this characteristic is more distinctive in adults).
The cowpea aphid is a sap-sucking feeder and damage caused in alfalfa by this pest results from the injection of a toxin into the phloem of the plant. With high population densities on plants, the aphid can cause stunting or plant death. In addition, it can cause yellowing in alfalfa leaves (Figure 1). Like other aphids, this insect produces honeydew that will benefit fungus growth and eventually cause sooty mold.
How to find them? Cowpea aphids are usually found in clusters on the alfalfa leaf and stems (Figure 4). It can also be found in vegetative growth and flower parts of a wide range of hosts. They are readily sampled with sweep nets.
Figure 4. Cowpea aphids on growing tip of alfalfa at BARC. Photo courtesy of Russell Griffith.
Host plants. Cowpea aphid is most commonly found in alfalfa, but may be found on other legumes, such as clovers. More uncommonly, the aphids occur on a variety of weeds and other plants in other plant families.
Management options. Unfortunately, there is not an economic threshold specified for this pest in Maryland alfalfa at this point. However, here are general guidelines for responding to the pest:
Conserve natural enemies. Natural enemies such as lady beetles, damsel bugs, and parasitoid wasps often locate, feed, and reproduce in conjunction with high densities of aphid in alfalfa. If you conserve natural enemies you might find aphids parasitized by parasitoid wasps (Figure 5). When scouting for aphids, watch for natural enemies to help control aphid populations. To help natural enemies stay in your alfalfa field you can use border-strip cutting while harvesting to provide refuge habitats. For more information of this practice, see “Harvest Scheduling and Harvest Impacts on IPM” at the end of this report.
Monitoring for decision-making. Early infestations in alfalfa can result from migration from southern areas. Pay attention to alfalfa fields in March and continue to monitor until fall. Since there are no thresholds developed for cowpea aphid, the thresholds for insecticide applications developed for the blue alfalfa aphid can be used: if alfalfa is 10 inches, then treat if there are 20 or more aphids per stem; if alfalfa is 20 inches tall, then treat if 50 or more aphids per stem.
Figure 5. Aphids parasitized or aphid mummies by a parasitoid wasp. Note the emergence hole (yellow arrows) of the parasitoid wasp in the mummy. Photo courtesy of Darsy Smith.
Potential reasons for the outbreak.
Researchers in the Lamp Lab, University of Maryland, have noted this pest in greenhouse settings but rarely observed them in alfalfa fields. Dr. Lamp suggests that the cowpea aphid may have migrated into Maryland because this species is more common in southern and western areas. Also, the mild winter may have allowed individuals that migrated last year to overwinter in Maryland. Additionally, lack of natural enemies in early spring can potentially lead to an outbreak. Conserving the natural enemies in alfalfa fields and neighboring areas can help decrease aphid abundance.
How can you help?
Records from your alfalfa fields and surrounding crops are valuable sources of information. The information is helpful to not just explain an outbreak but also to provide useful guidelines for farmers to manage the crop and avoid a future outbreak. Practical information that you can provide include any of the following:
Date of first time you have observed the pest
Date of outbreak
Plant height and phenology stage: when you observed the pest for the first time and during outbreak
Presence of natural enemies
Pesticide use and efficacy of application
Alfalfa cultivars/varieties planted during outbreak and previous year
Pictures of damage and estimate of loss
If you find cowpea aphid in your alfalfa field, please contact the nearest county extension office.
Bill Cissel, Extension Agent – Integrated Pest Management, University of Delaware; bcissel@udel.edu
Congratulations to Bob Leiby for correctly identifying the parasitized aphid in the photograph and for being selected to be entered into the end of season raffle for $100 not once but five times. Everyone else who guessed correctly will also have their name entered into the raffle. Click on the Guess the Pest logo to participate in this week’s Guess the Pest challenge!
The aphid in the photograph has been parasitized by a tiny wasp, about 1/10th inch in length. The adult female wasp, referred to as a parasitoid, lays an egg in the aphid. When the egg hatches, the tiny white parasitoid larva develops inside the aphid which eventually kills the aphid. Once the parasitoid larva finishes its development, it pupates, causing the aphid body to turn tan or black depending on the species of parasitoid. An aphid that has been parasitized is referred to as an aphid mummy.
Highly magnified image of a Braconid wasp (parasitic wasp).
Parasitic wasps and other beneficial insects play a major role in keeping aphid populations in check in many cropping systems. For example, in small grains during heading, a ratio of one beneficial insect (lady beetle larva, syrphid fly maggot, lacewing larva, damsel bug or parasitic wasp) per 50-100 aphids is often sufficient to achieve biological control. Because of this, if you have an aphid infestation, it is always important to also note beneficial insect activity.
Veronica Johnson, University of Maryland Department of Entomology
Aphids are small, early season pests that can occasionally reach damaging levels in small grain fields in Maryland. Strategies to control these insects should begin with correct pest identification and field scouting to determine infestation levels within a particular field.
Pest Identification:
Aphids are soft bodied, pear-shaped insects with piercing-sucking mouthparts and a pair of “tailpipe-like” projections, or cornicles, emerging from their lower abdomen. Adults can be winged or wingless, and the vast majority of aphids are female. A number of aphid species have been documented as either direct or indirect pests of wheat in Maryland. These include the bird cherry oat aphid, english grain aphid, corn leaf aphid, and the greenbug aphid. Continue reading Early Aphid Occurrences: A Possible Result of Warmer Winter Temperatures
Kelly Hamby, kahamby@umd.edu Assistant Professor/Extension Specialist, Department of Entomology
Ben Beale, bbeale@umd.eduExtension Educator, UME-St. Mary’s County
Sugarcane Aphid was found late last fall in Charles County, Maryland in a sorghum field that was being harvested for grain. Aphid populations were very high, with feeding present in the grain head and leaves. This is the first time that sugarcane aphid has been found in Maryland. While this aphid has caused substantial losses to sorghum in states to our South, it is unknown if the aphid will be present early enough and at high enough populations to cause significant injury in Maryland. Growers are encouraged to monitor sorghum fields through the summer for the presence of sugarcane aphid. We suspect sugarcane aphids are most likely to arrive later in the season in Maryland. Continue reading Sorghum Growers Encouraged to Keep an Eye Out for Sugarcane Aphid this Season
