Lavender is affected by several insect and disease pests but little is known about their development and life cycle on the crop. In 2015 we monitored four lavender sites over the growing season for all major and minor pests to track their development over the season. Knowing when pests emerge in a lavender field can help growers with their pest management. Currently there are few to no registered pesticides or biopesticides for control of insects and diseases of lavender. However, it is unknown if insecticidal soaps such as Trounce and Opal, which are registered for aphid control on lavender would have an effect on some of the insect pests such as garden fleahopper and four-lined plant bug. New products may be registered over time and timing their application will be key to their success. Most insect control products need to be targeted to young nymphs in order to be effective, and knowing when that will occur in a season is important.
The four sites were all located in the Simcoe, Ontario area. Timing of pest development may differ in other areas of Ontario. The major pests of lavender were four-lined plant bug, garden fleahopper, and Septoria leaf spot, but several more minor pests were also identified including spittle bug, tarnished plant bug, leafhoppers, and woolly bear caterpillars. Three plants in each of six random areas on each site were monitored. Plants were shaken to scare off insects into a sweep net placed under the plants. Each plant was then examined closely for diseases and less mobile insects. Actual insect counts per plant would be higher than identified in this project.
Four-Lined Plant Bug (FLPB)
Four-lined plant bug can cause significant cosmetic damage to developing lavender flowers at bloom. Scouting began on May 21 and FLPB nymphs were already at peak on that day (Figure 1). It is likely that emergence would have been a week or two earlier. Scouting will start earlier next year. Numbers steadily declined as they reached the adult stage in mid-June. There is only one generation of FLPB per year and adults were no longer found after July 10. Unfortunately their most damaging stages coincide with developing flowers. While the peak numbers were just above one insect per plant, populations would have been much higher.
Figure 1. Four-lined plant bug populations over the growing season at four sites in the Simcoe, Ontario area in 2015.
By mid-June we were able to find egg clusters on the lavender stems (Figure 2). The eggs were inserted in a slit in the side of green stems (Figure 3) and were very difficult to find. It is possible that many of the egg clusters are cut off the plants during harvest or pruning activities. Some of the egg clusters will be laid on the green stems coming up from the base of the plant, so it is impossible to remove them all. It is unknown if these egg clusters would die on pruned materials or if the pruned materials would have to be removed from the field to provide any control.
Figure 2. A FLPB egg cluster on a lavender stem. Note the small white vertical line on the green stem just below the leaf in the centre of the picture.
Figure 3. Closeup of a FLPB egg cluster inserted into a lavender stem.
Garden fleahoppers can reach very high populations and affect growth of lavender plants with their extensive feeding. Two very distinct generations of garden fleahopper were identified during the monitoring period with the first generation beginning at the start of the monitoring period on May 21 (Figure 4). The generations were clearly visible on most sites at relatively the same time for each generation. Peak numbers were over 25 per plant with actual numbers probably much higher. Garden fleahoppers tended to be patchy in the field with some plants at over 100 insects captured, while others at the same site had none. It is possible that a third generation emerges in September, but this is unknown from our study.
Figure 4. Garden fleahopper populations over the season in four lavender sites near Simcoe, Ontario in 2015
Septoria Leaf Spot
Septoria leaf spot is a perennial issue in lavender. It is unknown what impact it has on plant growth but it is likely that at least some weakening of the plant occurs. Septoria was identified at low levels starting at the first assessment May 21 (Figure 5). The disease was rated on a scale of 0 to 5 with 0 being no disease and 5 being over 80% of leaves affected. Disease pressures generally built over the season with the exception of two periods – just before bloom and in mid-August. Disease ratings probably declined just before bloom due to rapid growth of flowers and also declined in mid-August due to rapid vegetative growth after bloom. Septoria usually is worst on older leaves so these periods of rapid vegetative growth dilute the disease on the plant, at least temporarily.
Figure 5. Average Septoria leaf spot ratings at four lavender sites near Simcoe, Ontario in 2015
Other Insect Pests
Leafhoppers were found on all sites but at very low levels. There were three distinct generations during the study. One generation was just ending when scouting began on May 21. One generation peaked at the end of June and one began in late August. The leafhoppers were not identified to species and caused only minor feeding damage to the leaves.
Spittle bugs were also identified at all sites. They are usually a cosmetic issue only and have no impact on plant growth. There was one distinct generation at all sites beginning in late May and continuing to around the 1st of July. This coincides with the bloom period of angustifolia cultivars.
Tarnished plant bug nymphs were also identified at all sites. They were confirmed to cause damage on plants in the lab and were able to complete their life cycle on lavender. However there were no signs of obvious damage. If they do cause damage it is relatively minor and not noticeable on the plants. Nymphs were present at the beginning of scouting on May 1. They declined during June and then reach a definite peak at three of the sites in mid-July. A third peak may have just begun in late August right at the end of the scouting period.
Other pests included woolly bear caterpillars and grasshoppers, but these did not appear to cause major damage to the plants. Grasshoppers probably caused some chewing damage but at very low levels.