The secret life of weevils

Shining light on a nocturnal pest. By Anna Mouton.

Weevils — kalanders in Afrikaans — are a group of vegetarian beetles. There are nearly 100 000 species of weevils. Most live blameless lives but a few troublemakers cause problems by damaging crops and complicating international trade. Fresh Quarterly spoke to Prof. Pia Addison of the Department of Conservation Ecology and Entomology at Stellenbosch University for an update on weevil research co-funded by Hortgro.

A team of scientists led by Addison studied weevils in apple orchards in Ceres and Grabouw from spring 2016 to autumn 2019. Weevils are active at night. During the day they hole up in dark places — such as under the cardboard bands that the researchers placed around the trunks of apple trees. The researchers collared the weevils sheltering under the cardboard and hauled the miscreants to the laboratory for counting and identification. They also collected data on everything from orchard weeds to soil texture.

On the menu for weevils

The banded fruit weevil — Phlyctinus callosus — is by far the most common weevil in apple orchards. No surprise then that it accounts for most of the damage caused by weevils.

Banded fruit weevils are native to the Western Cape from where they were first described back in 1834. The preferred food of adults is the bietou or bush-tick berry — Osteospermum moniliferum — and they are seldom found anywhere else in the wild. But it’s a different story in man-made habitats where they are happy to eat a wide range of plants — banded fruit weevils especially love plants in the daisy family.

Weevil larvae feed on a large variety of plant roots. In orchards they usually munch weed roots. Addison’s team hoped to better understand the association between weevils and different types of ground cover.

“We struck it unlucky because we had a drought,” says Addison, “so for a lot of the time there were insufficient cover crops growing even in winter.” The most common weed in the orchards studied was the hairy wild lettuce or false dandelion — Hypochaeris radicata — which is known to be attractive to weevils. Plantains — Plantago lanceolata — were also associated with the presence of banded fruit weevils.

Getting to grips with weevils

Adult weevils lay eggs at the soil surface or on leaves and detritus. The eggs hatch in one to two weeks and the larvae plunge into the soil to devour roots. Eventually they pupate in chambers in the upper soil layers. Adult banded fruit weevils start to emerge from October and numbers peak in November. A smaller peak can occur in late summer.

“They’re not active in winter,” states Addison. “They only have these two activity peaks.” Damage to fruit is usually worst when adult weevil numbers are highest. Growers are advised to monitor from the middle of October. Monitoring is based on damage assessments and checking under cardboard bands placed around the trunks of trees.

Addison believes that biological control will in future play an important role in weevil control. Trials have shown that banded fruit weevils are vulnerable to Beauvaria bassiana — a fungus that attacks insects. “The larvae are the most susceptible, but it also works for pupae. If you spray early in the season, it’s best for the fungi and those pupae that emerge will come through the soil and get infected as adults.”

Formulations of Beauvaria bassiana are commercially available although not all are registered against weevils. The fungus also infects other pests including red spider mite and thrips.

Research on biological control is ongoing and both fungi and nematodes are being tested. “We have nematodes that work really well so if there’s a new pest, we first try those,” says Addison. “We’re trying to make a magic bullet — but it’s not always possible.”

Addison’s research team included Meshack Magagula — he completed a master’s degree working on weevil diversity and ecology — and Dr Bonginkhosi Dlamini — he gained his doctorate studying biological control of weevils under Prof. Antoinette Malan of the Department of Conservation Ecology and Entomology at Stellenbosch University. Matthew Addison, programme manager for crop protection at Hortgro Science, and Dr Julien Haran, research entomologist at CIRAD, collaborated on the research. They worked closely with new master’s student Steffan Hansen.

What are people saying about this research?

Bekker Wessels | ProCrop

“For years our number one insect pest on apples and pears was codling moth. Today it’s definitely weevils. It’s the single pest that causes the most fruit damage on most apple farms. It’s also a problem on stone fruit — especially nectarines — in certain areas. Depending on pest pressure, weevils can be the most expensive pest to control because the measures are expensive. The spray applications are expensive because the chemicals are very expensive. The primary method of control remains trunk bands and these are expensive to install and maintain both in terms of materials and labour. So any research that helps us control weevils is useful.”

Image: Banded fruit weevil — Phlyctinus callosus.

Supplied by Julien Haran | CIRAD.