Description of the Pest
The yellow and white alfalfa caterpillar butterflies lay eggs on new alfalfa growth. Eggs hatch into green caterpillars in 3 to 7 days. Full-grown caterpillars are about 1.5 inches long and are distinguished from other caterpillars on alfalfa by their velvety green bodies with white lines along their sides.
Alfalfa caterpillars overwinter as larvae almost entirely in annual vetch at low altitudes and colonize alfalfa only as the vetch senesces in May and June. Aside from alfalfa and annual vetches, the butterfly also breeds on a variety of clovers and sweet clovers and occasionally on lupines. Caterpillar outbreaks usually result from a flight of butterflies into the field when the alfalfa is less than 6 inches tall.
Alfalfa caterpillar swarms appear to be triggered when growers cut and bale alfalfa when adult numbers are high, with outflow in all directions. Extremely large numbers of adults migrating between fields are often present from June to September in the Central Valley and from May to October in the southern desert. Factors contributing to economically significant caterpillar numbers are
- Slow and uneven crop growth
- Lack of natural enemies
- Hyperparasites (other parasitoid wasps attacking the natural enemy wasps reducing their numbers)
- Hot, dry weather.
There are four to seven generations per year of alfalfa caterpillar, and each generation is closely synchronized with the hay-cutting cycle so that the caterpillar pupates before cutting occurs.
Damage
Alfalfa caterpillars can consume entire leaves. The larger larvae are most destructive. In contrast to armyworms, alfalfa caterpillars do not skeletonize leaves and will also consume the midrib.
Management
The most important way to control the alfalfa caterpillar is to preserve natural enemies that parasitize and prey upon this pest. Use selective insecticides against caterpillar pests in the summer to maintain natural enemies and minimize subsequent build-up of caterpillars. Preserve and encourage natural enemies by avoiding unnecessary insecticide applications for aphids or weevils in the spring.
Biological Control
An important parasite of the alfalfa caterpillar is Cotesia medicaginis, a dark brown to black wasp about 0.25 inch long. This wasp stings very small alfalfa caterpillars and lays an egg inside. The egg hatches and the wasp larva eats the inside of the caterpillar. A parasitized caterpillar dies before it reaches 0.5 inch in length. A parasitized caterpillar is
- lighter in color than normal,
- somewhat shiny rather than velvety on the surface, and
- swollen toward the rear.
Grasping the caterpillar at each end of the swelling and pulling it apart will expose the shiny, white parasite. It is important to determine the amount of parasitism because the economic threshold takes parasitism into account. For more information on identifying parasitized caterpillars, see the video in the ALFALFA CATERPILLAR AND ARMYWORM MONITORING section.
Cultural Control
Border-strip harvesting is a useful method for preserving the natural enemies of both the alfalfa caterpillar and aphids because it helps retain parasite larvae in the field. (For more details, see BORDER-STRIP HARVESTING.) Early harvesting of fields infested with economically significant levels of alfalfa caterpillars kills a large number of caterpillars, preserves crop yields, and avoids reducing natural enemy numbers. Time this cutting to avoid serious damage, yet obtain satisfactory yield.
Organically Acceptable Methods
Biological and cultural controls, as well as sprays of Bacillus thuringiensis, are acceptable for use on an organically certified crop.
Monitoring and Treatment Decisions
(View photos of caterpillars)
In early summer start sweeping fields with adequate plant height 2 to 3 times per week to monitor for caterpillars.
Combine monitoring of alfalfa caterpillars with armyworm monitoring as described in ALFALFA CATERPILLAR AND ARMYWORM MONITORING. Count and record the number of healthy and parasitized caterpillars caught in your sweep net on a monitoring form.
If cutting is not practical or not scheduled soon after monitoring, apply an insecticide if there is an average of:
- 10 or more nonparasitized alfalfa caterpillars per sweep,
- 15 or more nonparasitized armyworms per sweep, or
- 10 or more nonparasitized alfalfa caterpillars and armyworms combined per sweep.
Pesticides and Natural Enemy Releases
Not all registered pesticides are listed. The following are ranked by their IPM value, with the most effective and least harmful to natural enemies, honey bees, and the environment listed at the top of the table. When choosing a pesticide, consider information related to water and air quality, resistance management, and the pesticide's properties and application timing. Use PestManage to compare summarized management options for different pests in the same crop. Always carefully read the label of the product being used and take all necessary precautions when handling pesticides.
| Rank | Active ingredient | Example trade name | Group | Group Order | MoA 1 | Amount per acre | REI (hours) ‡ | PHI (days) ‡ | Comments | Selectivity 2 | Bees 3 | Predatory mites 4 | Predators 5 | Parasitoids 5 | Residue duration 6 | Leaching(fish) 7 | Adsorbed runoff(fish) 8 | Solution runoff(fish) 9 | Leaching(human) 10 | Solution runoff(human) 11 | Last updated 12 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
|
|
Javelin WG # | Application Timing Varies (See UC IPM Pest Management Guidelines and Label) | 8 |
11A |
0.25–1.5 lb | 4 | 0 | Bacillus thuringiensis will give satisfa... | narrow | low | low | low | short | — | — | — | — | — | 04/2024 | ||
|
|
Vantacor | Application Timing Varies (See UC IPM Pest Management Guidelines and Label) | 8 |
28 |
1.2–2.5 fl oz | 4 | 0 | To reduce the development of resistance,... | narrow | low | low | low/moderate | short | — | — | — | — | — | 04/2024 | ||
|
|
Steward EC | Application Timing Varies (See UC IPM Pest Management Guidelines and Label) | 8 |
22A |
6.7–11.3 fl oz | 12 | 7 | Make no more than one application per cu... | narrow | — | low | low | moderate | low | low | intermediate | very low | low | 04/2024 | ||
|
|
Intrepid 2F | Application Timing Varies (See UC IPM Pest Management Guidelines and Label) | 8 |
18 |
Label rates | 4 | See label | Make no more than one application per cu... | narrow | low | low | low | short | very low | low | low | very low | low | 04/2024 |
Legend
- ‡ a b Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without personal protective equipment. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases, the REI exceeds the PHI. The longer of the two intervals is the minimum time that must elapse before harvest.
- # ↩ Acceptable for use on certified organic crops. Check with your certifier to confirm before application.
- 1 ↩ Group numbers for insecticides and miticides are assigned by the Insecticide Resistance Action Committee (IRAC). Insecticides with unknown modes of action are assigned mode-of-action group numbers (MoAs) that begin with UN. Rotate pesticides with a different mode-of-action group number, and do not use products with the same mode-of-action group number more than twice per season to help prevent the development of resistance. For example, the organophosphates have a group number of 1B; insecticides with a 1B group number should be alternated with insecticides that have a group number other than 1B.
- 2 ↩ Range of insect and mite groups affected by a pesticide. Broad means the pesticide affects most groups of insects and mites; narrow means the pesticide affects only a few specific groups.
- 3 ↩ Risk of harm to honey bees. For more information, see Bee Precaution Pesticide Ratings.
- 4 ↩ Risk of harm to predatory mites. Toxicities are generally to western predatory mite, Galendromus occidentalis. Where differences have been measured in toxicity of the pesticide-resistant strain versus the native strain, these are listed as pesticide-resistant strain or native strain.
- 5 a b Risk of harm to parasitoids and general predators. Toxicities are averages of reported effects and should be used only as a general guide. Actual toxicity of a specific insecticide depends on factors including the application rate, environmental conditions, and the life stage and species of a parasitoid or predator.
- 6 ↩ Length of time residue affects natural enemies. Short means hours to days; moderate means days to 2 weeks; and long means many weeks or months.
- 7 ↩ Risk of harm to fish from leaching, based on USDA Natural Resources Conservation Service Windows Pesticide Screening Tool (WIN-PST).
- 8 ↩ Risk of harm to fish from adsorbed runoff, based on USDA Natural Resources Conservation Service Windows Pesticide Screening Tool (WIN-PST).
- 9 ↩ Risk of harm to fish from solution runoff, based on USDA Natural Resources Conservation Service Windows Pesticide Screening Tool (WIN-PST).
- 10 ↩ Risk of harm to humans from leaching, based on USDA Natural Resources Conservation Service Windows Pesticide Screening Tool (WIN-PST).
- 11 ↩ Risk of harm to humans from solution runoff, based on USDA Natural Resources Conservation Service Windows Pesticide Screening Tool (WIN-PST).
- 12 ↩ Date information was last updated in the UC IPM Pest Management Guidelines.
UC IPM Pest Management Guidelines: Alfalfa
UC ANR Publication 3430
L.D. Godfrey, Entomology, UC Davis
E.T. Natwick (emeritus), UC Cooperative Extension Imperial County
P.B. Goodell (emeritus), UC IPM and Kearney Agricultural Research and Extension Center, Parlier
R.F. Long (emeritus), UC Cooperative Extension Yolo County
V.M. Barlow, UC Cooperative Extension Riverside County
I.M. Grettenberger, Entomology, UC Davis
M.M. Leinfelder-Miles, UC Cooperative Extension San Joaquin, Contra Costa, Sacramento, Solano, and Yolo counties
M.D. Rethwisch, UCCE Riverside Co. (Blythe)
Acknowledgement for Contributions to Invertebrates
D.R. Haviland, UC IPM and UC Cooperative Extension Kern County
C.G. Summers (emeritus), Entomology, UC Davis and Kearney Agricultural Research and Extension Center, Parlier
