In the past two weeks we saw very high armyworm populations in rice. Many fields had to be treated. However, the pyrethroids, common insecticides used against armyworms, did not provide good control. Applications of carbaryl did not reduce populations either.

Most likely, these products failed to provide good control because of the very high populations. In an average year, when an insecticide treatment is needed, the number of armyworms in the field is much lower than this year, and therefore control is much easier to achieve. Achieving good control with an insecticide application under very high population pressure is difficult, even with effective insecticides.

Another factor that contributed to the lack of control this year is the size of the worms. As with most moth larvae, the smaller the worm, the easier it is to kill it. However, targeting small worms in rice can be challenging.

After hatching, armyworms go through six instars, or stages, before pupating and turning into moths. Each stage occurs when the worms outgrow their skin, and need to molt so that growth can continue. The first two instars stay hidden in leaf sheaths or inside furled leaves, where they feed. These worms are really small, eat little and are hard to find. The third instars start feeding on leaf blades, but the amount of tissue they consume is small and the injury difficult to notice (see Table 1 below). It is during the last two instars, fifth and sixth, when armyworms consume the most foliage. Usually this is when the injury and worms are noticeable. Unfortunately, these worms are the hardest to kill.

Table 1. Time to develop and rice foliage consumption by each armyworm instar

Instar	Duration of instar (days) at 70o F	Leaf area consumed per instar (square inches)	Leaf area consumed (% of total consumed per larvae)
First	3.9	0.06	0.15
Second	3.1	0.12	0.29
Third	3.1	0.36	0.87
Fourth	2.9	0.90	2.18
Fifth	3.8	9.23	22.25
Sixth	6.3	30.81	74.26

At 70^o F, it takes 13 days from hatching for worms to reach the fifth instar. With the warm weather experienced in the past weeks (average daily temperature for the last two weeks of June was 75^o F in Willows), armyworm development must have occurred somewhat faster. Infestations probably started in the early part of June and went unnoticed until the worms reached a large size.

Fifth or sixth instar armyworm feeding on rice leaf.

Populations are now dwindling. In most areas worm density is decreasing or have disappeared all together. A second infestation might occur sometime around the boot and heading stages. The second infestation does not originate from the one we just experienced. Armyworms pupate in the soil, but in rice fields, when they drop to find a suitable place to pupate, they drown. This means that the worms seen earlier do not produce the moths that will lay eggs in rice later in the season. Nevertheless, we might see another severe outbreak if the conditions that favored the previous outbreak continue. Growers and PCAs should remain vigilant and try to identify infestations early to avoid direct panicle injury, which can occur during the armyworm second infestation.

I recently visited a couple of fields in Glenn County with severe armyworm injury. It seems somewhat early to be seeing this type of injury. Application of pyrethroids are not controlling these really high infestations. Fortunately, the really heavy injury is confined to corners and borders of fields. 

 

The challenge with armyworms is to detect them early. Usually, the presence of worms is not noted until severe defoliation is observed. By this time, the worms are large and hard to kill with any insecticide. The fields I visited were about 45 days, and had large worms in them, which means that the infestation probably started at least 2 or 3 weeks ago. 

 

Keep in mind that at this stage, rice can take quite a bit of defoliation before a yield reduction or delay in maturity is observed. If defoliation is higher than 25%, a treatment is warranted. Also, most of the armyworms you observe in the field now will drop off the plants to try to pupate in the soil, but will drown. So they won't be a source of moths for the infestation we see around heading. But the earliness and high pressure of armyworms now is an indication that growers need to be on the alert near the boot and heading stages to try to detect early armyworm infestations at that point. During booting and heading, armyworm injury can cause more damage because they can feed on the flag leaf or directly on the developing panicle.

For more information, visit the Rice IPM Guidelines here.

Armyworms are common in rice fields from mid-July to August. Two species can infest rice, the armyworm and the western yellowstriped armyworm. These insects build up their populations in alfalfa, other grains and grasses, and invade rice late in the season. The adults are moths that lay their eggs either on rice leaves or on broadleaf weeds within rice fields. After the eggs hatch, the small larvae begin feeding on rice foliage and/or panicles. Young larvae are hard to see, they can be less than a quarter of an inch long and their color mimics older rice foliage. Larvae go through a series of molts, growing to 1.5 to 2 inches. When they are ready to pupate, they drop to the ground in search for a good pupation site in the soil. In rice, they usually drown. I have seen armyworms pupate in the leaf sheath of older leafs at the base of plants at the water level, but this is uncommon.

Foliage feeding does not affect yield unless the armyworms consume 25% or more of the crop's foliage. It's not uncommon to walk a rice field and find areas where feeding is noticeable. In those areas, open the canopy and look near the water level. You might find armyworms resting there during the day. At night or when it's cooler, they will climb up the plant and continue their feeding. Take notice of the size of armyworms you see – if most of them are small, they will continue their feeding and a treatment might be needed. If you can't find any armyworms, they probably have already completed their cycle and there is no need to do a treatment.

Armyworm damage can be more problematic during heading. Typical panicle injury consists of blank panicle branches, although sometimes the whole panicle can be blank. What happens is that the armyworm bites the rachis of the branch and the grains in that branch don't fill. Sometimes, armyworm injury can be confused with cold temperature blanking. However, if you look closely at the blank panicle or branches, you will see a chewing mark where the blank area begins. Inspect your fields as the panicles come out of the boot – I believe this is when most of the armyworm damage occurs. If after inspecting your field you find that 10% or more panicles are affected, and you can still find armyworms, a treatment might be needed.

For the most part, armyworms are not a major problem in California rice. Some growers experienced severe infestations last year; heavy infestations seem to be cyclical and return every few years. Luckily, armyworms have several natural enemies in rice fields that are very good at keeping their populations down. Birds are important armyworm predators and can function as a warning system for their presence in the field.

 

I've been receiving reports of high armyworm populations this summer. You can always find some armyworms in rice fields, but I have rarely seen fields that needed a treatment due to armyworm injury.

Defoliation caused by armyworms is not uncommon. However, rice plants have abundant foliage and can take quite a bit of defoliation. 

UC guidelines recommend a treatment only if more than 25% of the foliage has been consumed and you still see armyworms in the field. 

It can be challenging to find armyworms in a rice field. They usually feed at night. During the heat of the day, they hide at the base of the plants where it's cooler. To monitor for armyworms, open the canopy and look down near the water level. Spend a minute or two looking; small armyworms can be hard to see.



Alternatively, you can shake the plants and observe for worms falling into the water. This is especially effective for large larvae.



Panicle damage can be severe. Armyworms can feed on developing panicles causing whole panicles or panicle branches to become blank. 



Observe closely where the panicle branch breaks or becomes blank and you will see chewing marks.



UC guidelines recommend a treatment if more than 10% of the panicles are affected and you still see worms in the field. If you can't find them during the day and are still in doubt, come out during the evening and see if you see any worms in the panicles.



Those fuzzy white cocoons you find at the base of the plants are pupal cases of the parasitic wasp Apanteles militaris. The wasp lays its eggs inside the armyworm, wasp larvae develop and when they are ready to pupate they leave the armyworm and form the white cocoons. This, of course, kills the armyworm.



It's important to remember that armyworms pupate in the soil. In a rice field, when they are ready to pupate, they usually drop to the water and drown. Some may pupate in the foliage at the base of the plants, but there they are easy prey for their natural enemies. If you see injury but the worms are gone, there is no need to worry about the next generation of armyworms.