Now is the time to start scouting your fields for Spring pest and weeds. This past week I have received several calls asking about aphid control in alfalfa, primarily the blue alfalfa and the pea aphid.
The blue alfalfa aphid is a large blue-green aphid with long legs and antennae. It is similar in appearance to the pea aphid but can be distinguished by examining the antennae. The antennae of the pea aphid have narrow dark bands on each segment, whereas those of the blue alfalfa aphid gradually darken to brown as you near the tip of the antennae.
Both the blue alfalfa aphid and the pea aphid prefer cooler temperatures (optimal temperature for development of both blue alfalfa and pea aphid is around 60°F); however, the blue alfalfa aphid is more tolerant than the pea aphid of cool temperatures and appears earlier in growing season. Damaging numbers are most common in the spring to early summer in the intermountain areas. However, blue alfalfa aphids may also be found in the fall in many areas.
While feeding on alfalfa, the blue alfalfa injects a toxin that retards growth, reduces yield, and may even kill plants. The toxin injected by the blue alfalfa aphid is more potent than that of the pea aphid (pea aphid toxin itself is not particularly damaging to alfalfa plants). Toxin that remains in the stems and crown after harvest of the upper plant material may continue to retard stem growth and elongation and may carry over to the next cutting, or even the subsequent two cuttings.
Damage can also reduce the alfalfa's feeding value. A black fungus, sooty mold, grows on the honeydew excreted by the aphid and reduces palatability to livestock. Damage is more severe on short growth alfalfa than on taller alfalfa for both species.
Start to monitor fields as soon as the first aphids are observed. Economic treatment thresholds for both blue alfalfa and pea aphids are as follows (if both species are present, use the blue alfalfa aphid treatment levels): SEE GRAPH
The utilization of IPM (Integrated Pest Management) techniques is the best way to manage aphid populations. These includes planting resistant varieties, biological control, cultural control and chemical control.
• Resistant Varieties - Planting alfalfa varieties resistant to blue alfalfa aphid has been the most effective means of controlling aphids in alfalfa. Prolonged periods of below-normal temperatures, however, may lower plant resistance to blue alfalfa aphid when it is most needed and some crop injury may occur. At around 70°F, the host plant resistance should be fully expressed.
• Biological Control - The most significant aphid predators are several species of lady beetles that attack and consume both the blue alfalfa and pea aphid species. Green lacewings can also be important in regulating aphids and many other predators including bigeyed bugs, damsel bugs, and syrphid fly larvae also play a role.
• Cultural Control - Use border-strip cutting during harvest to help maintain populations of parasites and predators within the field.
• Chemical Control - If natural enemies fail to keep the aphids in check, an insecticide application may be necessary. The following pesticides are recommended for aphid control:
FLUPYRADIFURONE (Sivanto 200SL) and (Sivanto Prime)
COMMENTS: Do not apply more than 28.0 fl oz of Sivanto Prime or 200SL (0.365 lb flupyradifurone)/acre per calendar year on alfalfa, regardless of product or formulation.
FLONICAMID (Beleaf 50SG)
COMMENTS: Use allowed under a 24c registration (SLN CA-140006).
CHLORPYRIFOS* (Lorsban Advanced)
COMMENTS: Do not apply when bees are present. Avoid drift and tailwater runoff into surface waters. Preharvest interval is 7 days for cutting and grazing when 0.5 pt/acre used, 14 days for 1 pint/acre, and 21 days for rates above 1 pint/acre.
24 See comments
DIMETHOATE (Dimethoate 2.67EC)
COMMENTS: Preharvest interval (PHI) is 10 days for harvest or pasturing; for alfalfa seed: do not feed or graze livestock on treated crops, hay threshings, or stubble within 20 days of application. Check label to see if product allows only one application per year or per cutting.
Do not apply when bees are present.
48 See comments
METHOMYL* (Lannate SP)
COMMENTS: Highly toxic to bees: do not spray directly or allow drift onto blooming crops or weeds where bees are foraging. Can also kill natural enemies.
LAMBDA-CYHALOTHRIN* (Warrior II with Zeon)
COMMENTS: Preharvest interval (PHI) is 1 day for forage and 7 days for hay. Can be disruptive to natural enemies. Highly toxic to bees; do not spray directly or allow to drift onto blooming crops or weeds where bees are foraging.
1.28–1.92 fl oz
24 See comments
COMMENTS: Preharvest interval (PHI) is 3 days for cutting or grazing and 7 days for harvesting seed. Can be disruptive to natural enemies. Highly toxic to bees; do not spray directly or allow to drift onto blooming crops or weeds where bees are foraging.
2.4–4.3 fl oz
12 See comments
*See label for dilution rates.
For more information, refer to UC Pest Management Guidelines, Alfalfa
Blue Alfalfa Aphid at http://ipm.ucanr.edu/PMG/r1302311.html
Source: UC IPM Pest Management Guidelines: Alfalfa UC ANR Publication 3430