Plant Doctor Archive

What's bugging my hibiscus?

I HAVE a hedge of 24 different hibiscus varieties, now about nine years old and around 1.5m high and wide. They have two problems:

1. There are two to three green shield beetles on the flower stem just under each bud, causing the buds to drop off without blooming. What can I do to get rid of them?
2. In the centre of the blooms, on mostly the lightcoloured flowers, there are large numbers of very small, hard-backed, match head-size black beetles. What can I do about them?

SHIELD bugs can affect a wide range of plants, with green vegetable bug being the most common. They suck sap from leaves, stems and fruit and it's this damage which causes the flower bud drop on your hibiscus.

Apart from catching them by hand and squashing them, which is hardly practical with a hedge, the safest way to control them is to spray with one of the organic pyrethrin-based sprays such as Garlic & Pyrethrum or Nature's Way Pyrethrum. You'll need to make sure you actually get the spray on to the bugs for good control. Otherwise you could use a systemic spray such as Confidor or Rogor 100, which gets into the plant sap stream and is ingested by the bug as it feeds. Next summer be on the lookout for shield bugs early in the season and spray as soon as you see the first few to knock the population down before it has a chance to reach problem-causing proportions.

The black beetles in the flowers sound like pollen eaters - they are quite common on roses and other ornamentals in some seasons. They don't harm plants, they just crawl into flowers to feed on pollen, so you won't need to take any action against them. However, spraying for shield bugs will almost certainly affect them too.

Reproduced with permission from the former Weekend Gardener magazine. The views expressed here are not necessarily those of the RNZIH.