- Author: Luis Espino
- Author: Randall Mutters
The armyworm outbreak we experienced a few weeks ago seems to be over. Several fields in Glenn and Butte counties had very high populations and experienced severe defoliation. In some cases, the affected areas have recovered (see pictures below). However, there are reports that a few fields were defoliated to the water line. As we approach PI and topdressing, it is worth thinking about the likely consequences of the outbreak.
Above, severe armyworm defoliation. Below, same area, 20 days later
Rice has the capacity to recover from defoliation by producing new leaves and tillers. When the main tillers are affected, the plant will try to compensate for the loss in foliar area by producing additional lateral tillers. These tillers will mature later and will delay the maturity of affected areas of the field respect to less affected areas. In these fields, expect uneven grain maturity during harvest.
Nitrogen may help in the recovery of defoliated plants. In some cases, growers have applied nitrogen to the affected areas in an attempt to promote rapid regrowth. However, excess nitrogen can have several undesirable consequences. Excess nitrogen promotes stem rot and blast, diseases that can affect plants all the way until panicle maturity. Also, too much nitrogen can extend the plant's vegetative growth period, resulting in delayed development and uneven grain ripening. When topdressing in the next few weeks, avoid over-fertilizing armyworm affected areas.
In fields that have been severely affected, water management may also be complicated. Flooding periods may have to be extended, and with current water conditions, this will put extra demands on an already stressed water system. If possible, plan on turning off the irrigation and allowing the water to subside instead of draining the field. In fields with heavy clay soils, it is safe to not have standing water (soil still saturated) 24 days after 50% heading (when one-half of the panicles in a field have emerged) without risking yield loss and grain quality.
Be on the lookout for another armyworm infestation by early August. Most years, there is a second population peak 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 other causes, such as cold temperature blanking. However, if you look closely at the blanked panicle or branches, you will see a chewing mark where the blanked area begins.
Armyworm panicle injury during heading
Armyworm panicle injury later as seen during grain maturity
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. Monitor your fields constantly to catch those infestations as early as possible.
Small armyworm feeding on rice panicle
- Author: Luis Espino
Recently, a grower had been observing several straw colored moths flying around his field. He worried those could be armyworm moths that were laying eggs. Turned out those moths were not armyworm moths. Below are a few pictures of what the armyworm moth looks like.
As you see in the pictures, the armyworm moth is big and stout. These moths are nocturnal, flying and laying eggs at night. I have seen them flying out of the rice foliage when disturbed by walking through the field. But otherwise, they are hard to find.
- Author: Luis Espino
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 70o 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 75o 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.
- Author: Luis Espino
UCCE Rice Production Workshop
An in-depth workshop that covers the principles and practices of rice production
Friday, July 24, 2015
The Refuge Restaurant
1501 Butte House Road
Yuba City, CA 95993
Program:
8:30 Sign in, pick up class materials
9:00 Introduction and Workshop Overview
9:10 Rice Growth and Development
9:30 Land Formation, Water Management
9:50 Tillage, Planting and Stand Establishment
10:10 Break
10:30 Variety Selection
11:00 Invertebrates
11:30 Diseases
12:00 Lunch
1:00 Fertility
2:00 Weeds
3:00 Adjourn
Enroll on-line at http://ucanr.edu/2015riceworkshop
Prepaid enrollment ($100) is required, and enrollment is limited to 75 people. Please enroll by July 21st to ensure your participation. Seats will be filled on a first-come basis.
DPR (2.5 Other) and CCA (1 Nutrient Management, 2 IPM, 1 Crop Managment) continuing education credits available.
For more information, contact Cass Mutters (530-538-7201), Luis Espino (530-458-0578), or Michelle Leinfelder-Miles (209-953-6120), Farm Advisors, UC Cooperative Extension.
- Author: Luis Espino
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.
