Hadula trifolii (clover cutworm)

Indications of attack by H. trifolii result from external feeding by larvae on leaves of the host plant; holes and notches appear on the leaves. However, because these symptoms also appear as the result of the action of quite a number of other species' external feeding larvae, it is difficult to make a diagnosis based only on these.

Cultural Control

Some cultural methods might theoretically reduce H. trifolii damage, but have limited applications in major commercial crops, where there is little scope to alter cultural practices, such as changing planting or harvest dates.

Biological Control

Parasitoids and pathogens of H. trifolii are abundant throughout the whole of its distribution range, and most likely only a small fraction of them are known. In many areas, these natural enemies generally maintain H. trifolii population numbers below pest threshold levels. However, whenever and wherever this does not happen, there is a potential for artificially enhancing the population numbers of one or more of these biological control agents to reduce H. trifolii populations. However, specific biological control methods have, so far, not been undertaken against H. trifolii.

H. trifolii seems to be especially harmful to sugar beet crops in eastern Europe, according to the data of Puzuirnuij (1931) from the Ukraine; Dochkova (1971), Thko (1972) and Subchev et al. (1987) from Bulgaria; Zolotov (1982) from Russia; Szeoke and Szendrey (1997) from Hungary; and Lipa (1977) from Poland. However, quantitative data on the crop losses caused by H. trifolii on sugar beet is not available. Because other noctuid larvae also feed on sugar beet at the same time as H. trifolii, it is often difficult to obtain reliable data. For example, according to Lipa (1977), in 1976 an outbreak of cutworms occurred in the areas around Szczecin, Poznan and Wroclaw, Poland. On many plantations of potatoes, sugar beet, red beet, leeks and other plants, densities of 2-6.7 larvae/m² were observed. The predominant species was Agrotis segetum, which accounted for 50-91% of all species collected. Larvae of Agrotis exclamationis accounted for 8-33%, and in some regions the larvae of H. trifolii accounted for 16%.According to Lange et al. (1978), the main insect pests of sugar beet grown in California, USA, are two homopterans (Myzus persicae and Circulifer tenellus), vectors of some viruses, and a complex of lepidopterous larvae of which the dominant species are often Spodoptera exigua and H. trifolii. Since 1978, pest management schemes have been concerned mainly with preventing the spread of the viruses. H. trifolii, and the other lepidopterans, appeared to be of less importance, because artificial defoliation experiments showed that beet plants could sustain considerable loss of leaf area without loss of yield.In Canada, H. trifolii is generally a sporadic pest (Ayre et al., 1982), but the larvae can cause damage to a wide variety of plants, including many vegetables, field crops and some deciduous trees (Tietz, 1972). Ayre and Lamb (1990) recorded crop losses from this species in Manitoba in 1980, when damage was found in sunflowers, oilseed rape, onions and flax. In one 64-hectare field of flax, in which the entire crop was destroyed, as many as 160 fifth- and sixth-instar larvae were found per square metre. From 1980 to 1987, larvae were found in small numbers in a variety of crops, but no economic damage was reported again until 1988. In that year, numerous fields of oilseed rape and flax, in an area extending approximately 100 km north of the Manitoba-USA border and from the Red River to the Manitoba-Saskatchewan border, were treated with insecticide for the control of larvae. The 1988 light-trap catch was more than 3-fold that previously recorded. However, in 1980 when crop damage was also recorded, the light-trap catch was lower than in years when no economic damage was reported.Also in Canada, H. trifolii has been reported as the eighth major insect pest on rape, after five species of flea beetles (mostly Phyllotreta cruciferae), bertha armyworm (Mamestra configurata) and diamondback moth (Plutella xylostella). However, damage produced by H. trifolii larvae only occasionally becomes serious (Madder and Stemeroff, 1988). According to these authors, during the period 1980-85, the only hectarage treated against H. trifolii in Canada was in Alberta (10,000 ha in 1982 and 1,000 ha in 1983). Foliar-applied insecticides were relied on to reduce H. trifolii numbers; insecticide plus application costs for its control were $15.71/ha. These authors assessed the costs and benefits of insect control on rape during 1980-85 using estimates of crop losses from researchers, extension personnel, agrochemical companies and growers. Concerning H. trifolii only, the total annual net benefits, after accounting for research, extension work, insecticide and application costs, were $445,600 ($44,56/ha) in 1982 and $56,200 ($56,20/ha) in 1983.H. trifolii has also been recorded as a minor pest of other crops, including cabbage in Romania (Roman et al., 1995) and Bulgaria (Tkho, 1972), lucerne in Turkmenistan (Alekseev, 1972) and Bulgaria (Tkho, 1972), clover in Bulgaria (Tkho, 1972) and maize in Hungary (Szeoke and Szendrey, 1997). It very rarely affects trees, though Styles (1960) mentions this species as a pest of young conifers in forest nurseries in the UK and Tietz (1972) quotes some deciduous trees in North America. However, objective assessments of crop losses due to this noctuid seem to be lacking.