Revisiting biological control of field bindweed
Field bindweed is a Eurasian vine whose dense creeping and twining growth smothers other vegetation and its long-lived seeds and deep roots make it hard to control. It is a noxious weed of agricultural fields in temperate regions and has become invasive in North America. CABI is studying sustainable control methods using host-specific natural enemies, which could be introduced into North America as biological control agents.
So, what’s the problem
Field bindweed (Convolvulus arvensis) is a perennial vine of Eurasian origin. The plant was introduced to other continents including North America and is now considered one of the most noxious weeds of agricultural fields throughout temperate regions. Forming dense tangled mats of vegetation, it outcompetes native forbs and grasses, and can severely reduce crop yields. It can harbour plant diseases and contains alkaloids that may be toxic to some grazing animals, in particular to horses. Its extensive root system and long-lived seeds make it difficult to control by conventional means. Biological control offers an alternative approach: one reason for the plant’s impact may be the absence of natural enemies that attack it in its area of origin.
The weediness of field bindweed is largely attributable to its extensive root system and North American insects attacking the leaves are having little impact. So the United States Department of Agriculture (USDA) initiated a programme to manage field bindweed using biological control in the 1970s. Two biological control agents were introduced from Europe: the gall mite Aceria malherbae and the bindweed moth Tyta luctuosa. The impact of the gall mite varies, while local establishment of the bindweed moth has only been reported from the western USA and reports from Oregon suggest low impact. The weed continues to be a problem and additional biological control agents are being sought.
What is this project doing?
The project is being revisited through an initiative set-up by the late Dr Richard Hansen (USDA-APHIS-CPHST). CABI’s centre in Switzerland is investigating additional potential agents. So far, we have studied five insect species that showed potential for biological control, and are planning work on two more species.
A guiding principle for biological control is that any released agent should not impact plants other than the target weed. Risk of potential non-target damage is assessed by testing whether a candidate agent feeds or develops on other plant species that it might encounter if introduced. Assessing the impact an agent might have on the target weed is also important so the most-damaging ones can be prioritised.
We have rejected two root-feeding flea beetles which proved insufficiently specific and two leaf-feeding species because leaf feeders already exist on field bindweed in North America.
Since 2011, host-specificity tests with the stem-mining agromyzid fly, Melanagromyza albocilia, have been conducted. In no-choice tests (offering one plant species at a time), it laid eggs on six species, including four native to North America; larvae were found on two North American (NA) Calystegia species, one of which (C. macrostegia) supported complete development to the adult stage. To assess its specificity under more natural conditions, since 2017, we started to expose native North American, or economically important plants, at natural field sites in southern Germany. So far, we have tested 15 species (13 NA natives and sweet potato). None of the species, accepted for egg laying in the lab, were attacked under field conditions. Besides C. arvensis, few larvae were found on the native NA Cal. longipes and only when wild field bindweed was growing intertwined with the test plant. We plan to continue these open-field tests with additional native NA or economically important plants.
In addition, we are investigating the root-mining clear-wing moth, Microsphecia brosiformis (syn. Tinthia), which is only recorded from C. arvensis. Larval feeding can cause the plant to dieback. Methods for rearing this moth are being developed and host-specificity testing is being conducted at the Institute for Plant Protection and Environment in Serbia. In no-choice egg transfer tests, none of the nine test plant species exposed (eight NA natives and sweet potato) were attacked.
Both species appear to be highly host specific and damaging and show good potential as biological control agents for field bindweed.
Institute for Plant Protection and Environment, Department of Plant Pests, Zemun, Serbia