- Author: Luis Espino
This year, armyworm infestations were not as severe as last year. Populations did not reach the very high numbers of 2015, but they were early. Growers and PCAs were scouting diligently, and were able to recognize infestations when the worms were small. This gave growers the upper hand and many were ready to make a treatment decision if necessary. Intrepid was used in some fields successfully.
We are not totally off the hook yet. We usually get a second peak of armyworm activity during the late boot and heading stages. UCCE is monitoring moth populations in several areas of the valley using pheromone traps. The traps were set up in early July, and the trapping numbers will be shared in this blog and the UC Rice On-line website.
So far, moth numbers are low, averaging 2.5 moths/trap/day during the week of July 11. Numbers have decreased since the previous week, when the average was 4.3 moths/trap/day. The location with the highest moth numbers is near Knights Landing, averaging 9.2 moths/trap/day. Previous work done by Larry Godfrey, UC Davis Extension Entomologist, found that peaks of 20-40 moths/trap/day might indicate an armyworm larvae peak 7 to 10 days later.
Number of moths/trap/day captured in armyworm pheromone traps across the Sacramento Valley
- Author: Luis Espino
In the past week, many growers and PCAs have identified fields infested with armyworms. The size of these worms is small, ranging from first to third instars. The amount of defoliation they cause at these stages is small. However, once these worms reach the fifth instar, they will consume a lot more foliage and have the potential to cause yield reductions.
Last year, infestations sneaked up on us and were difficult to control because of the number of worms and their size. Older worms are much harder to kill than younger worms. This year, the fact that many growers and PCAs have noticed the infestations early gives us the upper hand. Nevertheless, I would not recommend treating until you notice defoliation is approaching the threshold.
Many factors affect the survival of armyworm larvae in the field. There are many natural enemies that will consume the larvae, such as spiders, plant bugs, beetles, frogs, etc. Additionally, weather can also play a role. Humid conditions can promote the development of armyworm diseases. Cold weather will slow down their movement, making them more susceptible to attack by natural enemies.
Research has shown that rice can withstand up to 25% defoliation before a yield reduction occurs. Most of the time, defoliation is limited to discrete areas of the field. Monitor for foliar injury by looking for signs of armyworms feeding on leaves. To sample, choose a part of the field where you have observed injury. Select a plant at random and pull it up or move all the surrounding foliage away and check for defoliation. Check the plant from the top of the leaves to the base of the plant and the water surface for armyworms. Determine if 25% or more of the foliage has been removed by armyworms; also note if you find armyworms on neighboring weeds or rice plants. Repeat this procedure every 5 to 10 feet across a transect until 10 plants have been examined. Move to a different part of the field where feeding is evident and examine 10 more plants in the same manner. Repeat this procedure at several areas of the field until you are confident that you have an estimate of the average field condition.
Keep in mind that warm weather speeds up armyworm development, and therefore, defoliation will occur faster. Once you are approaching treatment levels, make the decision to treat so that there is enough time for the application to go out before the threshold is reached. Windy days and applicator backlog can delay the treatment date.
- Author: Luis Espino
Earlier this week, I received reports of two fields near Willows that showed evidence of armyworm feeding and very small worms in the plants. Today, I sampled a field that had some severe armyworm affected areas last year. At first sight, there was no evidence of injury. However, when looking closely, some of the bottom leaves in the plants showed evidence of armyworm feeding.
This is a 40-day old field, drained for propanil. When I shook the injured plants a bit, a small worm fell to the ground half the time. The worms were hard to notice; they were small, and their color ranged from yellow to dark green. Most of the injured plants and all the worms I found were in the corners of the field and next to levees, where the plants showed symptoms of N deficiency because the aqua rig couldn't reach.
The larvae I found ranged from first to early third instar. It is the fifth and sixth instars that will cause yield reductions (look at the red line in the graph below). It should take between 127 and 268 degree days (above 50 and below 84.2 oF) for the larvae in the field I sampled to reach the fifth instar. Using average temperatures for the past 10 years, I calculated that we should be seeing fifth instar larvae between June 15 and June 21. Those dates are really close to the dates when we saw the armyworms causing problems last year.
Start scouting for armyworms. Begin in the areas where plants are light green because of N deficiency (corners, levees). Small larvae do not cause yield reductions, but once they reach a large size, they can defoliate rice quickly. Additionally, the larger the worms are, the harder to kill with an insecticide. If you find small worms, it does not necessarily mean that they will reach economic levels. There are many natural enemies in rice that can reduce armyworm populations, such as spiders, wasps, and plant bugs. Weather can also affect the development of the worms. Monitor the field closely. Once defoliation is noticeable, plan a treatment soon while the worms are still small if you feel you are at risk of reaching the threshold.
For information on thresholds, see the UC IPM website.
- Author: Luis Espino
It is difficult to accurately estimate yield losses due to armyworm damage. Early infestations that injure only foliage can reduce tillering, delay the crop, and cause uneven maturity. Infestations during heading and grain filling injure the panicle, causing blanks and broken panicle branches.
If defoliation was severe, try to estimate the percentage defoliation and what proportion of the field was affected. Many of the affected fields I saw had small areas where defoliation was almost complete, and some areas where defoliation was small. For example, such a field would have 80% defoliation on 90% of the area of the field, and 10% defoliation on the remaining 10% of the area of the field.
For panicle injury, observe the panicles before harvest. Broken branches are relatively easy to spot (see picture below). Grab 10 to 20 panicles and count how many have broken branches. Repeat this in three or four areas of the field, and then transform it to % injured panicles. If more than 10% of the panicles were injured, the action threshold was exceeded and there may be a yield reduction. The higher the % injured panicles, the higher the yield reduction. Research has shown that for every 1% increase in injured panicles, there may be a 0.07% reduction in yield. So, in a field with 10% injured panicles, yield could be lowered by 0.7%.
It's important to keep these estimates, together with any records of insecticide applications and yield, so that if we are in a similar situation next year, the industry can justify a section 18 for the insecticide Intrepid again.
- 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