Four-lined plant bug (FLPB) nymphs have been reported on lavender in Ontario and are likely present on many perennial mint-family herbs such as mint, lemon balm, thyme, oregano, sage and winter savory at this time. Since the adults lay their eggs in the late spring, which hatch the following year, four-lined plant bugs will rarely attack annual mint-family herbs such as basil and rosemary.
FLPB are true bugs closely related to tarnished plant bug. They have a wide host range and can affect numerous ornamental and weedy species, especially those in the mint family (Lamiaceae) and the aster family (Asteraceae). The adults are green/yellow with four black stripes running the length of the body (Figure 1). The nymphs are brown/rust coloured in the first instars but become a bright red with black bands and wing pads as the nymphs grow larger (Figure 2).
Figure 1. Adult FLPB on lemon balm.
Figure 2. FLPB nymph. Earlier instars are a duller red/brown.
Unlike tarnished plant bug, FLPB pierce leaves and stems in multiple areas resulting in spotting all over the young tissues (Figure 3). When the nymphs are small and the damage occurs on newly developing leaves, the spots may result in twisting of the leaf and occur on the leaf edges (Figure 4 and Figure 5). This is the damage that can be seen now in lavender. The damage can resemble a foliar disease but the spotting is too uniform and the insects are usually present on the plants. However, FLPB nymphs and adults move very rapidly on the plant and will hide as you approach the plant making it difficult to find them. They can be found by approaching the plant slowly and scanning from a distance or rapidly moving the branches, scaring the insects into the open. FLPB have only one generation per year in June and adults are usually done feeding in early to mid-July.
Figure 3. Feeding damage from FLPB can look similar to leaf disease. The upper leaf is FLPB damage and the lower leaf is Septoria leaf spot on lavender.
Figure 4. Early feeding damage to developing lavender shoots. Spotting may be more irregular when nymphs are small.
Figure 5. Close-up of early FLPB damage showing twisting of leaves and some spotting.
There are no products registered for control of FLPB in these crops. Insecticidal soaps (Trounce, Opal) registered for control of other insects in most mint-family herbs including lavender may provide some suppression of FLPB. However, contact with the insect is required and this can be difficult to achieve because of their mobility within the plant. The insecticidal soaps should be applied in the evening. Removal of the egg masses later in June may be an option for control of next year’s insects, but it is unknown if this is practical on a large scale. Floating row covers may not be a good option for FLPB control, since the eggs are laid on the plant and the row cover would just trap the insect on the plant. In ginseng, the use of a mint trap crop has been effective for luring the insects out of the ginseng. However, damage has been seen on several mint-family herbs in the same field and it is unknown if there is one herb that is preferred over the others. Other practices that may reduce damage to lavender and herbs include removal of leaves and debris around plants in the fall, planting affected plants away from other mint-family herbs since it can take several years for new plantings to be affected, and control of weedy hosts.
For lavender, the damage will occur to the developing flowers and affect the only harvest. There is no major reduction in yield, but marketability can be significantly affected if the stems are used in the end product. For other herbs, affected shoots developing in June will be unmarketable but the damaged tissue can be pruned out once the adults are done feeding and the new growth will be unaffected through the remainder of the summer.
