Potential Problems with Alfalfa Weevil

Published: April 6, 2009

Alfalfa weevil larvae are present in many Southern Missouri alfalfa fields as eggs laid in late fall and early winter hatch and larvae emerge. The alfalfa weevil (Hypera postica Gyll.) is an introduced pest which arrived in Missouri during the mid-1960s. Adult weevils are light brown in color with a wide dark brown stripe running down the back. The adult beetles are approximately 3/16 inch in length and possess the typical snout-nose associated with the weevil family of beetles. One generation is produced annually with adult weevils overwintering and actively laying eggs when temperatures rise above 50OF for several consecutive days during these seasons. Alfalfa weevil larvae are green to lime green in color with a white stripe running the length of the back. The head capsule is black to dark brown in color.

This insect is the primary insect pest of alfalfa statewide, but especially in southern counties where two or more peaks of larvae may be produced depending on when eggs were laid during the previous fall, over the winter, or in early spring. Eggs may be deposited in stems of overwintering stubble or in the stems of actively growing alfalfa plants during early spring. Alfalfa weevil eggs develop and eventually hatch after accumulating about 300 degree day heat units based on 48OF. This means that infestations of alfalfa weevil larvae often occur first on south-facing slopes of alfalfa fields, because these slopes warm faster in spring. Once emerged, larvae grow through 4 worm stages or instars before spinning a pupal case around themselves. Within a few weeks larvae change into adult weevils and emerge during late spring. Emerging adults usually leave alfalfa fields for the summer to hide in cooler field border areas before reentering fields to lay eggs in the fall, winter, and spring seasons.

Alfalfa producers in the southern counties of Missouri should actively be scouting alfalfa fields at least weekly or even twice weekly to determine pest numbers and plant damage. Producer in Central and Northern counties should begin scouting for alfalfa weevil within the next 1-2 weeks and continue through removal of first cutting harvest. The first damage observed will be small feeding holes in alfalfa leaflets as they grow out of the terminals of plant stems. This minor foliage damage is caused by the 1st and possibly 2nd larval (worm) stages called instars. As larvae grow larger (3rd and 4th instars) they consume greater amounts of plant foliage and may cause severe economic loss.

Scouting for alfalfa weevil is accomplished by randomly collecting 50 alfalfa stems (10 stems at 5 different locations) and tapping them into a white bucket. Larvae will generally be dislodged by this action and allow for an average number of larvae per alfalfa stem to be calculated. Caution should be used when collecting stems as larvae can be easily dislodged from the growing tip of the plant stem by rough handling. It is recommended that the top of the alfalfa stem be cupped in one hand while the plant stem is removed by cutting with a knife near the base of the stem. If an average of one or more larvae per stem is found, then the economic threshold has been reached and control is justified.

Management Options:
(1) The main management option for early infestations of alfalfa weevil larvae on small alfalfa is an application of a labeled insecticide when larvae reach or exceed threshold levels. If an insecticide application is selected, a list of insecticides recommended for alfalfa weevil control follows. Rates are given as amount of formulated product applied per acre.
(2) Early harvest of the alfalfa by either machine or livestock may be viable options for some producers in Missouri. If early harvest of alfalfa by machine is selected as a control strategy, then the crop is harvested approximately 7-10 prior to the normal plant growth stage of 1/10nth bloom. Data from Missouri indicate that alfalfa weevil larval numbers are reduced by about 98% with mechanical harvest and about 90% by cattle grazing in a management intensive grazing system. Producers using grazing as a control strategy must be aware of the bloat risk to cattle grazing green alfalfa and risk to the alfalfa stand due to trampling during wet conditions.

Scattered problems with Cowpea and Pea aphids in alfalfa have been reported from southwest Missouri. The Cowpea aphid (Aphis craccivora) is a very dark gray to black aphid which was first found in Missouri in the early 1990's. Cowpea aphids are the only black aphids inhabiting alfalfa in Missouri. Adult Cowpea aphids are relatively small in size, shiny black in color with dull white appendages, whereas nymphs (immature aphids) tend to have dark gray coloration. This insect tends to feed on the tips of alfalfa during early spring and can cause yellowing of plant leaflets from the bottom upward and possible stunting of plants. Although no formal economic thresholds for Cowpea aphid have been calculated from Missouri infestations, thresholds developed by the University of California – Davis suggest thresholds of 10 - 12 or more aphids on stems of alfalfa plants 10-inches or less in height and 20 - 40 or more aphids on the stems of alfalfa greater than 10-inches in height justify treatment. Missouri thresholds for pea aphids (Acyrthosiphon pisum) in alfalfa indicate than an average of 50 or more pea aphids per alfalfa plant may justify treatment of the pest population on plants up to 20-inches in height and 100 or more aphids on plants greater than 20-inches in height. If plants are under drought stress or growing slowly due to cool weather, then the threshold number would be reduced. Treatment also may be justified if plants are yellowing and aphids are present. The pea aphid is larger, green in color, and can be identified by a dark band around the base of the antennal segments. Pea aphid problems are most severe on slow growing alfalfa during early spring. Later infestations of the pea aphid during spring may cause economic problems, but generally plants 10-inches or more in height can withstand higher numbers of aphids. As with Cowpea aphids, pea aphids can cause yellowing and sometimes wilting of plants due to the removal of plant juices. The tables that follow list insecticides labeled for control of these pests in Missouri. Be sure to follow all label directions, precautions, and restrictions.

Wireworm Baits and Preplant Decisions for Corn

Wireworm is a group of insects which are often difficult to scout and manage. One method used to determine wireworm numbers prior to planting is the use of a solar baiting system. It can effectively estimate wireworm larval populations present at a site.

The scouting technique consists of placing bait stations or traps at several locations within a crop field. A minimum of two bait stations per acre is recommended, but in reality establishing 5 to 10 bait station per 30 to 40 acres of crop field should be sufficient if traps are properly located. In order to gain accurate estimates of the wireworm population, traps should be located in high risk areas such as in any grassy areas of the field or in areas where wireworms caused injury in previous seasons. Although trap placement in fields may occur 2-3 weeks prior to planting of the corn crop, traps placed 7-10 days prior to planting provide more accurate estimates of wireworm numbers as wireworms often remain deep in the soil until soil temperatures warm in the spring.

This trapping technique consists of digging a 4-inch deep by 6-9 inch wide hole at the soil surface. Place into the hole 2 to 1 cup equal mixture of untreated corn and wheat seed which has be pre-soaked for 24 hours prior to use in order to speed up seed germination. Fill and slightly mound each station with soil. Cover each mound with an 18-inch square of black polyethylene plastic (appropriate sized trash bag) followed by a 1-yard square sheet of clear polyethylene or similar clear plastic bag. Cover the edges of the plastic layers with soil to prevent wind damage. The black plastic layer absorbs heat and the clear plastic helps retain heat in the soil producing a "greenhouse effect" which allows for more rapid germination of the bait seed. Carbon dioxide is produced during the germination process and attracts wireworms to the bait. Just prior to planting, remove the plastic layers and soil from the bait and count the number of wireworm larvae in and around the bait. If the average number of wireworm larvae collected in all baits located in the field average one or more per bait station, the economic threshold has been exceeded and treatment is justified. If an economic infestation is found, control options implemented before or at the time of planting are recommended. Management options include such strategies as use of liquid or granular insecticides applied at planting or high rate insecticide seed treatments. Rescue treatments for this soil inhabiting insect pest are usually not effective for this pest. High infestations of this pest may result in severe stand loss, occasionally requiring the replanting of corn. An insecticide option should be used if a wireworm damaged field requires replanting. Most wireworm species found infesting corn typically live for 3 to 5 years as larvae before pupating into adult click beetles. These beetles generally live one season during which time they mate and lay eggs in the soil, often next to grass plants.