Researchers from ENDURE’s Hungarian partner, Szent István University, have gained valuable experience in participatory Integrated Pest Management (IPM) training for farmers after the arrival in the country of the destructive western corn rootworm (WCR), a pest that causes substantial crop losses in maize in North America every year.
Here Judit Papp Komáromi and Zoltán Pálinkás , of Szent István University’s Plant Protection Institute, and independent adviser István Terpó share their knowledge of WCR and the lessons they have learned in seeking to minimise its impact in Hungarian agriculture.
“This pest will teach us what IPM really is,” was the summary of one Hungarian farmer after finishing a one-year participatory IPM field training programme on western corn rootworm (WCR, Diabrotica virgifera virgifera LeConte), in Hungary, proving to us that combating a serious pest can have an upside.
The ‘story’ of WCR in Europe is a relatively recent one, beginning in 1992 when larval damage was first detected in a small maize field near Belgrade airport in Serbia. The detection of an invasive pest is always important news, but the fact that WCR is one of the most important maize pests in the United States Corn Belt made this discovery a headline event: it was the first time it had been reported outside America.
How did WCR become a pest?
WCR is the number one maize pest in the US Corn Belt but that was not the case many decades ago and there are two theories of how the host-pest relationship for WCR and maize has evolved:
Either theory could be right. WCR was not a pest in the past and has often been described as a man made-pest, since human interventions to control it have resulted in the pest becoming stronger.
After WCR damage was first detected in 1909 in Colorado, it began a rapid spread across the USA. Farming practice aided this acceleration: continued maize production in the same fields allowed populations to build up and invade new areas. The main control method used in the Corn Belt of America was the application of insecticides, but for these to remain effective farmers found they had to first increase dosages and then switch to other insecticides (with other active ingredients), then to increase dosages and so forth. By the late 1960s more than 60% of the maize growing area of Illinois’ Corn Belt was being treated with insecticides to control WCR.
Biology and development of the pest
WCR has one generation per year. It belongs to the insect family of leaf beetles, such as the widely known cereal leaf beetle (Oulema melanopa). Most leaf beetles over-winter in the adult stage, though WCR does this in the egg stage in the soil, at a depth of 15-30 cm (female WCR adults prefer to lay their eggs in the soil of maize stands). First larval hatching starts in spring, mainly in middle to late May and onwards, when maize has already been planted in the field. There are three larval stages and larval development takes three weeks to complete. Pupation takes place in the soil and takes one week to 10 days. Adult emergence starts in late June or early July. The peak population of adult WCR varies from July to early August. Females start laying eggs from the middle of July in the soil of maize fields and adults remain active until the first frosts (these dates are for Central and Eastern Europe).
The biology of WCR is very much linked to maize stand and maize growth stages. WCR larvae can most successfully develop on maize roots. There are other grassy weed species on which WCR larvae can complete their development, but the mortality of larvae is high in non-maize host plants. WCR adults tend to remain in the maize field from where they have emerged or, during inter-field movements, they tend to move to other maize crop stands, thus egg laying mostly happens in the soil of maize fields. If maize is cultivated continuously in the same field in subsequent years, the WCR larvae feed on the root of maize plants, their development continues and the circle remains unbroken. Once we understand the biology of WCR, we can see that crop rotation is the best management option.
How did WCR lose its fidelity to maize?
To decrease insecticide applications in the USA, extension services increasingly advised the use of crop rotation. It’s worth noting that as soon as the first WCR damage was recorded, changes in cultural practices were recommended for its management. However, widespread adoption of a soybean-maize crop rotation created the conditions that favoured an existing, but rare, WCR variant with a reduced egg-laying preference to maize fields. Reduced egg-laying preference means that WCR females lay their eggs outside of maize fields - and in the US Corn Belt this meant in the soil of soybean fields. Consequently, if maize is planted in that field in the subsequent year, damage will appear. Significant WCR larval damage in first year maize was observed in 1987 in the USA.
Damage and symptoms
WCR larvae feed on the root system of maize plants. First and second stages of larvae consume hairy roots, tunnel from the root tips and can reach the plant base, leaving visible feeding scars. Third instar larvae generally feed on larger nodal roots near the plant stalk. Brace roots are often damaged as well. Due to larval damage maize roots become weaker and maize plants begin to lodge. When larvae finish their development, thus finish destroying the maize roots, a symptom known as maize stalk goose necking can be observed. Plants lodge in each and every direction, with rows becoming indistinct and even unobservable.
Adults feed on maize leaves, pollen, silk and young, milky kernels. Adults feed on maize leaves if pollen and silk are not yet available. They feed on the epidermis of maize leaves in linear streaks and as maize leaves grow, these weak tissues tear and leaves become fuzzy. WCR prefer silks and pollen to leaves, and during the silking period adults clip the silks. This is the point where adults can cause economic damage, since silk clipping before or during pollination can result in poorly filled maize ears. Adults can also feed on young, milky kernel seeds.
WCR adults can also feed on other crops and plants. Pollen and leaf tissue of 19 plant species (cultivated crops and weeds) has been found in the gut content of WCR adults in Europe. However, WCR does not cause economic damage in other crop stands, only in maize.
WCR in Europe
Since WCR larval damage was first recorded in 1992 in Europe, rootworms have continued to spread and populations have become well established in Central and Eastern Europe. There are European Commission Decisions and Recommendations and various national regulations to prevent WCR (eradication measures) and to limit (containment and suppression measures) the further spread and population build up of WCR across the European Union. Suppression measures are relevant in the regions where WCR populations are already established and the pest has become a feature of the agro-ecosystem. In these regions, eradication measures are useless and measures have to focus on the development of IPM for WCR.
WCR behaviour in Europe
As a first step, we had to define whether the behaviour, biology and ecology of the pest is the same as it is in the USA. Based on four years’ experiments (in Hungary, Croatia, Romania and Serbia) and farmers’ experiences in Central and Eastern Europe, it was discovered that only the wild type of WCR is present in Europe, despite the fact it is widely assumed that multiple transatlantic introductions of WCR have occurred in Europe. The presence of only the wild type of WCR means the best management option is crop rotation.
How to tackle WCR?
The general principles of IPM for WCR can be well defined, however these principles have to be adjusted to regional or local conditions. And this is the most important point in integrated management of WCR!
We know that if we rotate maize, WCR larvae will die. However, farmers’ responses to this option were, generally speaking, that in all cases they were not able or willing to rotate maize (due to economic and other reasons) and they tried to control it with insecticides.
There is no question that insecticides may help to manage this pest but there is more than one solution. Rotation, supporting natural enemies, irrigation and so forth, and also careful use of certain insecticides, can help manage this pest. However, what we experienced is that farmers should learn step by step in their own fields about this management process.
Farmers can be given dozens of leaflets, listen to presentations every year and receive lots of information on WCR management, but they do not ‘own’ this knowledge. More importantly, this knowledge cannot be imported from outside their own sphere. Farmers have to discover how WCR has become integrated into the local agro-ecosystem and based on this information develop locally feasible IPM strategies against WCR, including the consideration of socio-economic factors.
Following these principles, participatory IPM training for WCR was conducted in seven Central and Eastern European countries (Bosnia and Herzegovina, Bulgaria, Croatia, Hungary, Romania, Serbia and Slovak Republic) from 2003 to 2008 under an FAO (Food and Agriculture Organisation of the United Nations) programme funded by the Italian government. The aim (which was achieved) was to protect maize production from the losses caused by WCR through the development and implementation of IPM strategies by farmers, based on a sound understanding of local agro-ecosystems and with the protection of local biodiversity as the main element of sustainable agricultural production. Through this project farmers developed locally feasible IPM for WCR.
We are absolutely convinced that IPM for WCR can be developed and implemented in a region through the joint activities of farmers, advisers and researchers, all relying on farmers’ basic knowledge. The ENDURE project is also focusing on this issue, with the maize case study and maize-based system case study surveying and analysing maize cultivation practices and formulating recommendations for IPM for maize. Training activities will cover, among other things, field training for IPM development for various crops alongside advisers.
We understand that developing the capacity of farming communities is a long process, but are equally convinced of its efficacy.