Description of the Pest
The adult true armyworm is a nocturnal moth with a wingspan of about 1.5 inches and a single white spot in the middle of its buff-colored forewing. (See photo of adult Western yellowstriped armyworm for comparison.)
Females lay eggs on either rice or other grass species in or around rice fields. Eggs are laid in linear masses with the leaf tied around them in a roll, making them difficult to find.
Larvae go through 6 instar stages. Early stages are small, difficult to find, and consume very little foliage. As larvae grow, the amount they eat increases, and defoliation becomes apparent as larvae reach the fifth and sixth instars in early summer. Larvae feed predominantly at night or during cloudy days, spending the day hidden among tillers near the water level. Larvae pupate in 3 to 4 weeks during summer lodged between rice tillers. Moths emerging from these pupae may lay eggs in rice again.
Two true armyworm generations occur in rice; moths fly in spring and early summer, laying eggs in rice fields and surrounding vegetation. The larvae that emerge feed on rice foliage. A second moth flight occurs in mid-summer, followed by larvae feeding on rice foliage and panicles.
It is important to distinguish true armyworm from western yellowstriped armyworm which can also be present in rice fields. Western yellowstriped armyworm moths only lay eggs on broadleaf weeds. Eggs form a flattened mass that is covered by body scales. Larvae feed primarily on broadleaf plant species and usually don’t cause economic damage to rice. The adult moth has mottled forewings and silver and gray hindwings.
Damage
Only true armyworm causes economic damage to rice. Injury is most serious during periods of stem elongation and grain formation. Larvae defoliate plants, typically by chewing angular pieces off leaves. In severe cases, all plant foliage can be consumed, leaving only a portion of the stem above the water. Defoliation greater than 25% during early summer will result in lower grain yields. During flowering and grain filling, larvae may also feed on the panicle rachis near the developing kernels causing these kernels to dry before filling. This feeding causes branches of the panicle to turn white and dangle from the panicle. Yield reductions have been documented if more than 10% of panicles are affected. Defoliation during flowering and grain filling does not typically result in reduced yields.
Management
Monitor throughout the summer to assess the need for insecticide application. Drowning and natural enemies kill many true armyworms in the rice field.
Biological Control
Many caterpillars are killed by natural enemies including predators, pathogenic microorganisms, and parasites. Natural enemies, especially when they limit spring and early summer generations in other crops and along field margins, often keep armyworms from becoming pests in rice. The wasps Hyposoter exiguae and Apanteles militaris are the most commonly seen parasites of the true armyworm. The larvae of these wasps live within the armyworms until they emerge to form white silk cocoons on tillers and leaves.
Viral diseases of armyworms are also important natural control agents under certain conditions of temperature and humidity. Diseased caterpillars first appear yellowish and limp. After dying, they hang from plants, appear deflated, and they ooze disintegrated body contents.
For information on protecting natural enemies, see Protecting Natural Enemies and Pollinators.
Cultural Control
Early weed control in and around rice fields may limit the development of armyworms infestations in rice fields.
Organically Acceptable Methods
Use cultural controls and naturally occurring biological controls in a certified organic crop. Bacillus thuringiensis ssp. kurstaki is registered for use in rice, and applications generally achieve 50 to 60% mortality of true armyworm.
Monitoring
Foliar Injury
Monitor for foliar injury during late June and early July by looking for signs of armyworm feeding on leaves. Once feeding damage is observed, monitor twice a week until larvae are no longer present.
To monitor, choose a part of the field where you have observed injury. Walk into the field and inspect plants in a small area around you from the water level to the top of the leaves. Check the water surface for armyworms that may drop from the plants. Determine if 25% or more of the foliage has been removed by armyworms and if they are still present. Record your observations. Repeat this procedure every 5 to 10 feet across a transect until 10 stops have been examined. Repeat this procedure in several areas of the field to create a confident estimate of the average field condition.
Panicle Loss
Monitor for panicle loss after panicle emergence by checking for broken panicle branches that have turned white, indicating possible armyworm feeding. Be sure to differentiate this injury from stem rot, cool temperature blanking, and rat damage, all of which may cause blanking.
Once armyworm injury to the panicle is observed, take samples twice a week to determine if an insecticide application is warranted. Inspect panicles in a 1-square-foot area and determine the proportion injured by armyworms. Inspect the plants down to the water level for the presence of armyworms.
Record the percentage of injured panicles and the presence or absence of armyworms. Move 5 to 10 feet and repeat the procedure until 10 samples have been taken. Move to other areas of the field with signs of panicle injury and repeat the process until you have a good estimate of the field condition.
Treatment Thresholds
During late June and early July, consider application of insecticides only in those areas of the field with more than 25% defoliation, and where armyworms are observed. If few or no armyworms are observed, come back to check in the evening when larvae are more active. Do not spray if armyworms are not present, especially during mid to late July, because they have probably completed development.
From panicle emergence to grain maturity, consider application of insecticide if 10% of the panicles in the area sampled are damaged and true armyworms are observed. If true armyworms are not observed but panicle loss is 10% or more, check for the larvae in the evening. If larvae are not found, do not spray because they have probably pupated and will do no further damage. Limit insecticide application to those areas of the field with economic damage.
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 | Predators 4 | Parasitoids 4 | Residue duration 5 | Leaching(fish) 6 | Adsorbed runoff(fish) 7 | Solution runoff(human) 8 | Leaching(human) 9 | Solution runoff(human) 10 | Last updated 11 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
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|
DiPel DF # | Application Timing Varies (See UC IPM Pest Management Guidelines and Label) | 8 |
11A |
0.5 lb | 4 | 0 | — | narrow | — | low | low | short | — | — | — | — | — | 04/2024 | ||
|
|
Dimilin 2L | Application Timing Varies (See UC IPM Pest Management Guidelines and Label) | 8 |
15 |
4–8 fl oz | 12 | 80 | Use permitted under 2(ee) label for ye... | narrow | — | moderate | low | moderate | — | — | — | — | — | 04/2024 | ||
|
|
Intrepid 2F | Application Timing Varies (See UC IPM Pest Management Guidelines and Label) | 8 |
18 |
8–10 fl oz | 4 | 14 | — | narrow | — | 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 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.
- 5 ↩ 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.
- 6 ↩ Risk of harm to fish from leaching, based on USDA Natural Resources Conservation Service Windows Pesticide Screening Tool (WIN-PST).
- 7 ↩ Risk of harm to fish from adsorbed runoff, based on USDA Natural Resources Conservation Service Windows Pesticide Screening Tool (WIN-PST).
- 8 ↩ Risk of harm to fish from solution runoff, based on USDA Natural Resources Conservation Service Windows Pesticide Screening Tool (WIN-PST).
- 9 ↩ Risk of harm to humans from leaching, based on USDA Natural Resources Conservation Service Windows Pesticide Screening Tool (WIN-PST).
- 10 ↩ Risk of harm to humans from solution runoff, based on USDA Natural Resources Conservation Service Windows Pesticide Screening Tool (WIN-PST).
- 11 ↩ Date information was last updated in the UC IPM Pest Management Guidelines.
UC IPM Pest Management Guidelines: Rice
UC ANR Publication 3465
I.M. Grettenberger, Entomology, UC Davis
L.A. Espino, UC Cooperative Extension, Butte County
S.P. Lawler, Entomology, UC Davis (General Information, Managing Mosquitoes in Agricultural Situations)
Acknowledgement for Contributions to Invertebrates
L.D. Godfrey, Entomology, UC Davis
