Given the more severe outbreaks of leaf folders (Zigzag Herpteogramma – Herpetogramma thestealis) over the past couple of years, we have been tracking their progress over the year to get a better understanding of their life cycle. This week, our research team, consisting of myself, Amber Witley, Summer Research Assistant, OMAFA and Amy Shi, OGGA, made a breakthrough in our understanding of the insects. We have found both the eggs of the insects and newly hatched caterpillars. The information below is preliminary, as we need to refine the details with further scouting. We are reporting this now because growers may need to apply insecticides to control the next generation of the insects.
The eggs are nearly impossible to see in the field, with a diameter of only about 0.8 mm and almost entirely transparent. They are laid on the bottom of leaves along the veins and appear as a shiny, circular spot that is only visible when the light catches it a certain way (Figure 1). Close up, you can see that they are circular, flat discs covered with a waxy coating (Figure 2). They are often laid singly, but are also found in lines of 2 to 4 eggs (Figure 3). Over time, the black head capsule of the caterpillar becomes visible inside the egg (Figure 4). Soon after, a small white caterpillar emerges with a black head and a distinct black, almost yoke-shaped band behind the head (Figure 5). The caterpillars feed on the underside of the leaves and begin to turn green, losing the dark markings on and behind the head. Figure 6 shows several different stages on the same leaf. This causes a ragged, window-like damage that is easier to spot than either the caterpillars or eggs in the field (Figure 7).
Figure 1. The tip of a ginseng leaflet showing a circular, mostly transparent egg next to the midrib. The egg is directly in the centre of the red circle.
Figure 2. A closeup of a single egg of the zigzag Herpetogramma leaf folder.
Figure 3. Two leaf folder eggs in a row.
Figure 4. An egg with the developing caterpillar visible inside. Note the distinct dark head capsule and band behind the head (see Figure 5).
Figure 5. A newly emerged leaf folder caterpillar with a distinct black head and yoke shaped band behind the head.
Figure 6. Several young caterpillars of different sizes, including 1) top left – a newly emerged white caterpillar, 2) top middle – early development after feeding for a day or two, and 3) bottom – a second or third instar caterpillar. Note the window-like feeding damage next to the younger caterpillars along with dark frass (insect droppings).
Figure 7. Window-like feeding damage caused by young leaf folder caterpillars feeding on the underside and unable to chew through the leaf at this stage.
It is presumed that this is the beginning of a second generation in the field that will cause further damage. These new caterpillars are much smaller (1.5 mm long at egg hatch) than the ones identified on newly emerging ginseng tops this spring. This likely means that this species overwinters as early instar (early stage) caterpillars. What we are seeing now is a smaller stage that we have never encountered before. Given how much of the summer still remains, it is highly likely there is a full second generation in the field, followed by adults re-emerging, laying more eggs, the eggs hatching, and then young caterpillars of that generation dropping to the ground to overwinter in or under the straw. However, it is also possible that the current caterpillars will drop to the ground now to overwinter. We will confirm which is correct with further monitoring.
Regardless of what happens with a second generation, now would be the time to spray to get good control if populations warrant it. If you have fields or sections of fields with significant damage from leaf folders this spring, it is important to scout these areas for the window-like damage to leaves. No other insects cause this type of feeding damage in ginseng. New leaf folds can also be found, but some of the young caterpillars appear to use the old leaf folds for protection initially. It is important to note that there are numerous beneficial insects in the field that could naturally control leaf folders and may be harmed by insecticide sprays. Our scouting indicated the presence of lacewing eggs and larvae and adult ladybird beetles (ladybugs), both of which are predatory on soft-bodied insects like small caterpillars. Plus, we have already identified at least 3 species of parasitic wasp that attack this species of leaf folder in ginseng fields. Here are the best strategies for getting control of this generation:
- Scout fields that had previous damage for the window-like damage
- Once the damage is found, determine if a spray is warranted based on the presence of window-like spots. There are no established thresholds for this pest, but control is probably warranted if 5-10 leaflets with symptoms are found on each metre (3 ft) of bed.
- Apply insecticides that will mainly target caterpillars. There are no products registered for leaf folders in ginseng, but products applied for control of leafrollers will likely also control leaf folders (Dipel 2X DF, Delegate, and Success/Entrust). Of these, Dipel and Success/Entrust are less harmful to beneficial insects. At this young stage, both of these should be effective. Cutworm products may also provide control, but many are broader spectrum and may harm beneficial insects. Read the label carefully for the best timing of application. Many of these products are effective when applied in the evening.
- Apply a different insecticide a week or two later because emergence is very staggered. Anything applied now may not have any effect on eggs that hatch over the next two weeks.
Once our monitoring is complete, we will be able to develop a better annual plan for control of this insect pest.