This year we initiated a small trial to determine if four-lined plant bugs prefer other hosts to lavender, which could then potentially be used as a trap crop to manage the pest. Four-lined plant bugs can cause extensive cosmetic damage to lavender stems and reduce their marketability, especially for flower bundles and crafts that require the use of the stems. Four-lined plant bugs are mainly pests of members of the mint family (Lamiaceae) and members of the aster family (Asteraceae).
We planted out four groups of four different hosts in squares, with the plants spaced 1 m apart. The four hosts were three members of the mint family (‘Grosso’ lavender, spearmint, and bergamot) and one member of the aster family (black-eyed susans). We planted them in May 2016 and confirmed that none of the plants had four-lined plant bugs prior to release of the adults.
On June 15 and 16 we noted that the insects had reached the adult stage in nearby lavender fields. We caught 52 adults and released 13 in the exact centre of each of the four squares of plants on June 16. One day later we can already see extensive feeding on all four of the spearmint plants (Figure 1) and on most of the black-eyed susans (Figure 2). There is only minor damage to one of the four bergamot plants. None of the lavender plants have any damage or insects present (Figure 3).
Figure 1. Four-lined plant bug damage on spearmint.
Figure 2. Four-lined plant bug adult and its damage (lower part of photo) on black-eyed susan.
Figure 3. ‘Grosso’ lavender showing no damage one day after adult release.
The experiment is nowhere near completion. There is still time for the adults to move around. They also may prefer different hosts for laying eggs, which they do by inserting them into vegetative stems. Egg-laying will occur over the next few weeks and they will not hatch until next year. We will also need to repeat it for a second year. We know that all of these hosts can support populations of four-lined plant bug, so it is likely that once the population is established it is less likely to lure them out of the lavender. It may be possible to attract the adults when they move into a new field for the first time and prevent them from going to the lavender. This still has to be tested.