The six-legged hitchhiker

The tiny grain pest catching a ride on our fruit. By Grethe Bestbier.

The grain chinch-bug — Macchiademus diplopterus — is a grain pest that feeds on cereals and grasses. This tiny critter has made a nuisance of itself by finding shelter among fresh fruit and catching a ride to wherever the fruit goes. Grain chinch-bugs do not damage the fruit, but they are a phytosanitary pest with costly implications for South African exporters.

A grain chinch-bug’s life

The grain chinch-bug is one of five species in the genus Macchiademus, all indigenous to the Western Cape.

Grain chinch-bugs prefer winter-rainfall areas and are most active in the cold season. During the winter they feed and reproduce on wheat or other grain crops. When summer arrives, adult chinch-bugs, with their dark bodies and silvery-grey wings, leave the wheat fields to find shelter for their summer dormancy. Bugs looking for a cosy and comfortable resting place may settle on fruit, settling at the stalk and calyx ends.

“Their preferred time of dormancy coincides with our fruit season’s harvest times. When wheat fields are close to orchards, and the fruit is ripe and being picked from the trees, the bugs might have settled on the fruit,” says Dr Shelley Johnson, Hortgro research associate in the Department of Conservation Ecology and Entomology at Stellenbosch University.

According to Johnson, grain chinch-bugs are not very selective about which fruit they occupy and will infest whatever orchard is near their grain hosts. Citrus, stone, and pome fruit are all viable options. Another popular hibernation spot is under the bark of eucalyptus trees.

“It basically looks for any little nook or cranny to go into,” says Johnson, who has been doing research on grain chinch-bugs since 2006. “Often, near grain fields, there are eucalyptus trees. The bark on eucalyptus trees comes off, so there are gaps behind it, and if you pull back the bark you can often find hundreds, or even thousands, of chinch-bugs.”

A little pest with a big impact

Grain chinch-bugs are a nightmare for exporters. When pack houses are close to wheat fields, the bugs are likely to enter, seeking shelter. Grain chinch-bugs are less than five millimetres long and can remain hidden in crates and packaging, creating the perfect situation for dispersion.

“These bugs are not cosmopolitan, meaning they are not everywhere. When they reach previously unexposed areas, they pose a real threat by adapting to the new conditions there, and cause damage to wheat plantations,” explains entomologist Prof. Pia Addison of the Department of Conservation Ecology and Entomology at Stellenbosch University.

The threat posed by grain chinch-bug to cereals has made it a zero-tolerance phytosanitary pest for many export markets.

According to Henk Griessel, quality assurance manager at Tru-Cape, the presence of a living grain chinch-bug in a packed product is not tolerated by European markets, and the fruit is rejected. South Africa’s other markets list grain chinch-bug as a phytosanitary risk and its presence can lead to the loss of that pack house or producer’s qualification to export. In worst cases, the export of a cultivar may be stopped.

“Grain chinch-bug has forced us to reconsider the export of Bon Chrétien and Beurre Bosc to the United States. They measure rejections over a 21-day period and if one exceeds the percentage, all pears are removed from the pre-trip inspections status,” says Griessel.

Control strategies

The grain chinch-bug presents some challenges when it comes to control. Not only are chemical pesticides not an option so close to harvest, but chemicals also often do not reach the fruit calyx, where the bugs are hidden.

Moreover, grain chinch-bugs are able to tolerate very high and very low temperatures during dormancy, which overlaps with our main export season. They are resistant to heat and cold treatments and are able to survive cold storage for long periods. The bugs upregulate heat-shock proteins in response to extreme temperatures, which allows them to tolerate temperature stresses. They also build up glycogen reserves to sustain themselves during dormancy so that they do not need to feed.

The best way to reduce the numbers that end up on fruit is to control the pest in the fields. Unfortunately, the damage to host crops often doesn’t justify control.

At the moment, preventive measures are limited, and post-harvest monitoring and control are the best options. “You need to determine whether you have a problem, and then prevent fruit from being exported if it is infested. The control strategy therefore moves to post-harvest divisions,” says Addison.

Johnson suggests a few strategies that fruit farmers could use to limit grain chinch-bug problems in fruit.

• If fruit orchards are close to fields, avoid picking from the rows closest to the wheat. A recent study showed that only the four rows nearest to the wheat crops are likely to have highly infested fruit. Although it is a difficult measure to implement, it is one of the best ways to reduce the numbers of chinch-bugs in fruit.
• Use an organic spray, for example a natural pyrethroid with a zero-days withholding period, in the orchard just before harvest, as well as in the pack house, to help control grain chinch-bugs.
• Prevent bugs from accessing fruit in the pack house by covering stocks of packaging material, such as crates and boxes.

Fumigation

A post-harvest treatment option gaining traction is fumigation with ethyl formate, an active ingredient used to smother and suffocate grain chinch-bugs.

“At the moment, fumigation with ethyl formate is the only post-harvest option for deciduous fruit, but the application thereof is challenging, and is something that we are still busy working on,” says Johnson.

Fumigation can be applied in the form of Vapormate, a non-flammable formulation of ethyl formate and carbon dioxide. Already used for treatment of grain and fruit produced in countries like Australia, New Zealand, and Israel, Vapormate is still in the process of being registered in South Africa, and presents challenges in terms of availability and affordability.

Recent research has focussed on developing a liquid ethyl formate fumigant with the aim of reducing costs. “We are looking at a liquid version that we can then just vaporise ourselves, using a nitrogen-rich environment with low oxygen levels, which is cheaper,” says Dr Renate Smit, postdoctoral research fellow in the Department of Conservation Ecology and Entomology at Stellenbosch University.

Thus far, their research has shown no phytotoxic symptoms on treated fruit. Smit stresses the importance of applying the correct dosage of the fumigant for grain chinch-bugs.

“You might have a concentration that kills chinch-bugs, but is not viable for the fruit,” says Smit. “We’ve done a lot of research to make sure which levels we can use, in terms of concentration, duration, temperature, and other factors, to determine when we are going to see phytotoxic effects. That is why an integrated approach is quite crucial to ensure sure that you kill the insect while the fruit quality remains intact.”

Johnson is confident that grain chinch-bugs can be beaten. “It’s been an issue and a difficult one for a long time,” she says. “We are still trying to solve the problem and find ways of applying preharvest control. We need to maintain market access, and to do so, we need to control grain chinch-bugs.”

Image: The loose bark of gum trees can shelter large numbers of grain chinch-bugs.

Supplied by Renate Smit | Stellenbosch University.