By Phyllis Bongard
This winter’s ample snow is good for alfalfa winter survival, but it can also favor insect pest survival. Drs. Craig Sheaffer, Extension forage agronomist, and Anthony Hanson, Extension IPM educator, had a wide-ranging discussion on the status of overwintering alfalfa and its pests in the March 15th Strategic Farming: Let’s talk crops session.
Alfalfa field covered with 4 inches of snow.
Outlook for overwintering alfalfa
Overwintering alfalfa is typically most vulnerable during March and April when a thaw is followed by freezing temperatures. However, the significant blanket of snow (>4 inches) in much of the state is providing good protection as the alfalfa comes out of dormancy. The snow insulates the plants from soil and air temperature variations that are typical this time of year. As a result, Sheaffer expects little risk of alfalfa winter injury. However, water from snow-melt can potentially lead to problems for alfalfa.
Ice sheeting is caused by melting snow that freezes on the surface and cuts off gas exchange to the alfalfa. If sheeting lasts for three to four weeks, it can cause significant injury and even death to alfalfa.
Since current soil temperatures are generally at or near freezing, there’s less risk for ice sheeting this year. If it does occur, it’s likely to be localized in low spots of the field. For those spots where ice sheeting is a frequent occurrence, leaving stubble in the fall can help break up the ice. Planting grasses that have more tolerance to ice sheeting, such as timothy or reed canarygrass, in low lying areas is a proactive option.
Heaving is another issue that can occur as we move into April. The freeze-thaw cycle can jack up alfalfa roots when soils are moist. If plants are raised more than an inch, there may be issues with plant survival and productivity.
Once heaving occurs, there is little that can be done. If heaving has affected a large proportion of the field, Sheaffer suggests rotating to corn. If only small areas of the field are affected, interseeding with ryegrass and red clover is an option. Reseeding alfalfa into an older stand is not recommended due to allelopathy.
Even though the risk for winter injury is low, alfalfa stands should be assessed in early May for any winter survival issues. For more information, see Alfalfa winter injury assessment and management.
Alfalfa weevil larva
Frost seeding is a low cost approach to introduce a new legume like red clover into pastures or into fall-seeded small grains. Once scattered on the soil surface, the freeze-thaw cycle will bury the seeds. The seeds will germinate and grow in the spring when temperatures are favorable. In addition to production of forage, legumes can contribute biologically fixed nitrogen for subsequent crops.
Be ready for spring seeding of alfalfa as soon as soil conditions are favorable. In many parts of the state, an April seeding will translate into additional forage yield the seeding year.
Soil testing is important for meeting all of alfalfa’s nutrient needs. Potash is one of the most important nutrients for alfalfa production and persistence. Be sure to apply at U of M recommended levels and not according to crop removal.
In addition to K, sulfur plays a critical role in alfalfa production. Reductions in atmospheric S deposition brought about by “clean air” legislation has led to sulfur deficiencies in many soils with low organic matter. If soils are low in organic matter, applying 10-25 lb of sulfur per acre fertilizer is an economical way to boost yields.
Alfalfa insect pest management
While snow provides good protection for alfalfa, it also helps insect pests survive. Using an integrated approach to control these pests identifies risks from both the pest and from pest management strategies, including personal health, economical, and environmental risks. Integrated pest management uses several “tools,” including insecticides, natural enemies, cultural control, and host plant resistance, to manage economically significant insect pests.
Keep in mind that beneficial insects can control some insect pests. If an “insurance” insecticide application is made, it will kill the beneficials which may also flare the insect pest population they were keeping in check. In addition, overuse can lead to insecticide-resistant insect populations and increase input costs when treatments aren’t needed.
Managing insecticide-resistant pest populations is a significant challenge. While pyrethroid resistance is well-documented among soybean aphid populations, it will likely become a concern for alfalfa weevil populations here, too. It’s already been confirmed in western states and insecticide failures were reported in Minnesota last year.
Rotating insecticide groups, avoiding repeated applications of the same insecticide in the same year, and spraying only when needed are important measures to maintain their efficacy.
Sporadic insect pests
Alfalfa is host to several insect species that occasionally cause issues. Examples include the variegated cutworm, plant bugs, pea aphid and blister beetles. Blister beetles concern horse owners, since the beetles contain a toxin that horses are sensitive to.
Insect pests that are sporadic generally don’t require regular scouting. However, two notable pests of alfalfa should be the focus of a scouting program: 1) Alfalfa weevil during the early season, and 2) Potato leafhopper during late season.
The alfalfa weevil is an invasive pest that was introduced to the U.S. in 1904. During the 1980s, a parasitic wasp from the weevils’ native range was introduced. It did a good job of suppressing the alfalfa weevil for a while, but it was also very sensitive to broad-spectrum insecticides.
The alfalfa weevil larva can be identified by its black head capsule and white stripe along its body. The cloverleaf weevil is often mistaken for the alfalfa weevil, but it has a brown head capsule instead of a black one.
The alfalfa weevil has one generation per year and overwinters as an adult. Once it emerges in May or June, it lays eggs in alfalfa stems. The larvae hatch and feed on alfalfa leaves for two to three weeks, going through four instar stages. After the last larval stage, the alfalfa weevil pupates, then emerges as an adult. Once they are adults, they enter summer dormancy and don’t cause any more feeding damage.
The larvae feed on alfalfa leaves between the veins, giving them a skeletonized appearance. Treatment thresholds are based on number of larvae – in addition to other factors – so scouting is essential.
Equipment needs are relatively simple. Use a 15-inch sweep net to check if larvae are present. If they are, then use shears to cut 30 stems across the field at ground level. Shake the larvae into a bucket and identify how many alfalfa weevil are present to get a count per stem.
The economic threshold (ET) is based on the number of larvae per stem, alfalfa height, value of the hay and treatment cost per acre. Alfalfa height affects treatment options when the ET is reached. For example, If the alfalfa is mid-vegetative (10-15 inches) at ET, an insecticide might be warranted. At the late vegetative stage (16-20 inches), mow or use an insecticide with a short pre-harvest interval. Finally, if more than 50% of the alfalfa is at early bud stage, it’s more beneficial to mow the crop.
Mowing and raking will damage larvae and expose them to the elements, as long as the hay can be baled quickly. Harvesting the crop as baleage or haylage will also get the alfalfa off the field and remove any protection for the weevil larvae.
With likely pyrethroid resistance in Minnesota, this group of insecticides is not being strongly recommended. If a field hasn’t had failure issues, careful use may maintain it as a useful tool. However, it’s likely that pyrethroid failures have occurred around the state.
With limited pyrethroid use, there aren’t many insecticide options. Group 1 insecticides include carbamates (1A) and organophosphates (1B). Organophosphates – malathion or phosmet – applied individually will achieve only about 50% control. However, control increases to about 90% with a combination of these two insecticides. One of the carbamates, carbaryl, can burn alfalfa after cutting.
The Group 22 insecticide, indoxacarb, is an effective option. However, last year it was in high demand and hard to find.
The limited number of insecticide options makes group rotation that much more important to reduce the risk of insecticide resistance. A new recommendation to avoid the same group within three years is making this even more challenging.
Historically, alfalfa weevil would be an issue for first cutting of alfalfa and possibly into the second. Hanson has started to see fields in western Minnesota where alfalfa weevil damage is extending into later June and into July, much later than what he normally expects. It’s possible that Minnesota is starting to see a new strain – the western strain – that is one to two weeks later than the predominant eastern strain.
Potato leafhopper is a migratory pest, arriving from the south each year. It tends to build through the season to become a late-season pest, although issues don’t occur every year. Leafhopper feeding damage can be identified by the V-shaped yellowing on leaves.
Resistant alfalfa varieties are available with no yield drag. Including grasses in an alfalfa stand will also help keep potato leafhopper numbers down.
Similar to alfalfa weevil economic thresholds, the ETs for potato leafhopper are based on alfalfa height, numbers of leafhoppers, and cost of treatment.
Several key practices will help maintain insecticide effectiveness..
- Use a variety of tools – including host plant resistance, natural enemies, and cultural practices to take the pressure off of insecticides.
- Scout to identify risk– Treat only when economic thresholds are reached. “Insurance” applications can do more harm than good.
- Rotate insecticide group numbers– Rotate groups at least annually and every three years, if possible, to maintain insecticide effectiveness.
- Use high labeled rates – Low labeled rates can increase the risk of developing insecticide resistance.
Join us for the final two sessions
Join us for the final two sessions of the University of Minnesota’s Strategic Farming: Let’s talk crops! webinar series. The March 22 webinar will focus on soybean insect pests with Extension Entomologist Bob Koch. On March 29, we welcome special guest Dr. Brian Luck, Extension specialist, Machinery systems and precision agriculture from UW-Madison, and Dr. Seth Naeve , UMN Extension soybean agronomist, to discuss getting your planter ready for spring. For more information and to register, visit z.umn.edu/strategic-farming.Source : umn.edu