This week’s article is an expansion on an earlier “Bug of the Week” post informed by Mike Bowie who was working at the Ministry of Agriculture.

The Chinese rose beetle (Adoretus sinicus) has been a common pest in the Cook Islands for many years. This beetle is a big problem for crops and ornamental plants, feeding on a wide variety of ornamental and food plants.

The beetle is originally from Japan and Taiwan but has been introduced to South East Asia where it is widespread. It now also has a presence across the Pacific islands, where it causes damage to both farms and gardens.

The Chinese rose beetle feeds on over 250 different plant species, including many important crops like taro, broccoli, cabbage, corn, green beans, sweet potato, and fruits such as star fruit, ginger, eggplant, and jackfruit. Its ability to feed on so many plants makes it a real challenge for farmers in the Cook Islands.

Adult Chinese rose beetles are small, about 10 to 12 millimetres long. They are oval-shaped, reddish-brown in colour, and covered in fine hairs that give them a greyish appearance.

They damage plants by eating the leaves, usually along the veins, which causes the leaves to break down and fall off. When there are many on a plant, they can quickly strip it of its leaves, leaving the plant weak and unable to grow properly.

They are nocturnal, meaning they feed mostly at night. They come out about an hour after sunset and can often be seen flying towards lights. During the day, they hide in the soil, under tree bark, or in piles of leaves. The beetles are strong fliers and can travel long distances in search of food.

Their larvae are white grubs that look a bit like the huhu grubs of New Zealand, but smaller.  They grow up to 12 millimetres long, live in the soil, compost, or leaf litter. While the larvae are not as harmful as the adult beetles, they might feed on young seedlings, which could cause damage to newly planted crops. The beetles’ life cycle, from egg to adult, takes about six to seven weeks, with much of that time spent underground as larvae.

Controlling the Chinese rose beetle is tricky, as the larvae spend most of their time in the soil. Digging up the soil can expose the larvae to birds and chickens, which may help reduce their numbers, but this isn’t always practical for larger farms. For smaller gardens, picking the beetles by hand at night can work, but it isn’t a user-friendly solution.

In Hawaii, researchers found that treating the plants with neem oil could reduce beetle damage for about four weeks. However, attempts to introduce natural predators, like parasitic wasps, have not been very successful in controlling the beetle. This is an area of ongoing research.

A closely related species, the rose beetle (Adoretus versutus), is also found in the Cook Islands. It looks similar to the Chinese rose beetle and behaves in much the same way, making it harder to tell them apart and manage them effectively.

While the Chinese rose beetle is not causing widespread damage right now, it remains a problem for farmers in the Cook Islands.

As research continues, better ways to control the beetle may be discovered to help protect crops and reduce the damage it causes.