I was evaluating armyworm injury in plots and noticed there was quite a bit of panicle blanking not caused by armyworms. When armyworms injury panicles, they feed on the rachis of panicle branches, causing those branches to dry out. Sometimes the kernels in those branches may be partially filled, but since they stop receiving nutrients and water due to the biting injury of the armyworm, they dry and turn straw colored. Most of the time, the branches brake off at the point of injury and can be seen hanging on the panicle. If you look closely at where the rachis is broken, you can see the biting mark of the armyworm.
The other blanking I was noticing was a little different. It was mostly on the panicles under the canopy, although some could be seen in the panicles on top. Blanked branches turned white, almost as if bleached, and kind of translucent. This might be because these panicles were young. Most likely they will dry out and also turn straw color. None of these injured branches were hanging on the panicle and no biting marks could be noticed. I suspect this blanking was caused by high temperatures. Temperatures above 104 F during flowering dry the germinating pollen tube before fertilization and cause blanking. It seemed that some areas in the field were more affected than others.
Going back to armyworms, I found very little injury in my Butte County trial. Two trials in Colusa had no injury. The number of armyworm moths caught in the traps as of 8/24 have come down again. I suspect we won't see the numbers climb up again, therefore the risk of armyworm injury now is very low.
Click on the images below to see a close up of armyworm injury (left) and high temperature blanking (right).
Number of moths trapped per day is increasing. Numbers are higher in areas where crop diversity is higher. I haven't seen any worms yet, but they are expected by the end of the month. At this point, moths may be mating and laying eggs on weeds and other plants in the area, including rice. This week I received a picture from a PCA of what seemed to be armyworm eggs laid in a rice seedling. No worms are expected on rice until late June or early July. In 2016, worms were observed in fields only after catches were around 20 monts/trap/day.
I have set up seven armyworm pheromone traps in the Sacramento Valley, and Michele Leinfelder-Miles is monitoring one trap in the Delta. Last year the traps showed a distinctive peak mid August, when worms started causing problems during heading. This year, we will start monitoring earlier and so we will try to identify the peak that corresponds to when worms start feeding on foliage in late June - early July.
I will publish the results of the monitoring weekly or biweekly on the UC Rice On-line website and I will sound the alarm when monitoring efforts need to be increased to make sure infestations don't go unnoticed.
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
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.
For information on thresholds, see the UC IPM website.