Cultural Control
Deep ploughing (to a depth of about 25 cm) after spring wheat harvest or after one shallow ploughing, is an effective method of controlling the population of H. tritici nymphs overwintering in the soil and may reduced numbers by 83.9-92.7% (Han and Xu, 1982).
In China and the former USSR, crop rotation is practised. The abundance of wheat thrips was 2-4 times lower in rotation crops than in monocultures (Antorenko, 1983).
Lower densities of H. tritici were recorded in winter wheat fertilized with nitrogen alone or with a mixture of nitrogen, phosphorus and potassium (Krasilovets and Sem'yanov, 1981).
Host-Plant Resistance
Varieties of wheat in which the last stage of booting or the initial stage of heading is earlier than the peak occurrence of H. tritici adults should be selected (Han and Xu, 1982). Krasilovets and Rabinovich (1979) reported that late winter wheat varieties were 2-4 times more infested than early varieties. Shurovenkov and Mikhailovna (1975) reported that the degree of damage by H. tritici to the grain was greatly affected by the length of the growth period, especially the length of time between milk ripeness and complete ripeness. Information was provided showing that late winter wheat varieties were 2-4 times more infested by H. tritici than early varieties in Ukraine (Krasilovets and Rabinovich, 1979).
In Bulgaria, research showed that the wheat varieties which had a shorter vegetation period were attacked to lower degrees and showed lower loss than wheats that matured later (Veselinov, 1976). A close relation was found between the closeness of the ears and the degree of damage. Ears with a close structure, having little space between the scales, were less heavily infested than ears with a more open structure (Shurovenkov and Mikhailova, 1978). When selecting wheat varieties resistant to H. tritici, hard glumules, tightly covering the developing caryopsis and compact ears are characteristics which should be sought.
Bournier and Bernaux (1971) found that H. tritici caused less damage to the grains of hard wheats. The damage caused in the furrow, produced when the wheat thrips penetrate beneath the glumules and puncture the cells of the pericarp during the milky stage of ripeness, is the most important. The selection of varieties of hard wheat with characteristics that prevent H. tritici from penetrating under the glumules appears to be the most promising way of preventing this type of damage.
Biological Control
The effect of natural enemies on field populations of H. tritici is rarely noticed because usually only a small proportion of each population is attacked.
Dyadechko et al. (1971) reported that two predators, Aeolothrips intermedius and Paratinus femoralis were frequently used in reducing numbers of H. tritici eggs and nymphs in Ukraine. In Kiev, Russia, H. tritici is only 4-5 times as numerous as Aeolothrips intermidius. At this ratio of host to predator, marked damage to crops does not occur and other control measures may be unnecessary.
Lewis (1973) indicated that most effective H. tritici natural enemies attack the nymphs and eggs.
Field Monitoring and Economic Thresholds
Nefedov (1948) estimated the degree of injury to spring wheat by comparing the weight of 1000 grains of different samples with different thrips populations infesting them. The weight of 1000 grains of the sample containing the minimum population of thrips nymphs served as his standard of comparison. The examination was done at the end of the milk stage or the beginning of the wax-ripe stage of spring wheat.
Tanskii (1958) took the following symptoms as indicators of the degree of grain infestation: weak infestation - imperceptible widening of the furrows of the grain only in the region of the beard and the presence of brown patches; medium infestation - deepening and widening of all furrows, brown coloration in their depths and bright areas noticeable in the places punctured by the nymphs; severe infestation - grain deformed and a considerable part of the outer covering bright in colour, due to punctures made by nymphs, in addition to the distension and deepening of furrows; very severe infestation - grain underdeveloped and undersized with deep wrinkles and folds.
Taking into consideration the difference between the weight of uninfested grains and that of grains with different degrees of infestation, estimates of total grain loss have been made. According to data obtained by Tanskii the loss in weight with a very mild infestation amounted to 5.6-7.0% and with a severe infestation, 15.9-31.6%. This method of estimating the damage done by thrips to grain crops gave low values since relatively uninfested grains were used as control and the damage caused by the insects until the grain ripened was not taken into consideration. A study of the damage to crops by thrips under the natural conditions prevailing in southern Ukraine was carried on for four years. The results revealed a certain relationship between the number of thrips entering an ear at the stage of blooming and the consequent loss of grain weight. It was shown that when the average number of adult thrips per ear were 3-6, 7.2-12.6, 13.1-19.7, 20.2-29.8, 30.6-41, then the loss of grain weight in percentage terms were 3.4-6.7, 7.2-9.4, 9.8-14.3, 14.5-24.5, 25.1-30.7%, respectively. The ratios between them were about 1:0.7-1.4:1 (Dyadechko, 1964).
In the Saratov region of the former USSR, during 1978-1980, tests were carried out in irrigated wheat fields to determine the injuriousness of H. tritici and the threshold of numbers at which this became evident. The results indicated that the danger from nymphs was not very high, though it was increased in the dry season of 1980. A threshold of 80 individuals per ear was established for spring wheat and this was seldom exceeded. Chemical treatment might be envisaged in some cases, but never later than the period of milky ripeness (Kamechenko, 1982, 1988).
A field study carried out in China in 1962 during the growth stages of winter wheat and spring wheat showed that from early May to early June, the number of adults at the last stage of booting or the initial heading stage, the number of eggs at flower stage and the number of nymphs at milk stage on winter wheat were 3.8, 36.2 and 14.5 per ear, respectively. The ratios among the adults, eggs and nymphs were 1:9.5:3.8. The number of adults, eggs and nymphs on spring wheat were 19.05, 104.44 and 81.98 per ear, respectively and the ratios among adults, eggs and nymphs were 1:5.48:4.3. According to this, if there are 18 adults per ear, then there may be about 77.4 nymphs per ear. The investigatory data in fields showed that when there were 18 nymphs per ear, then there was a 3% reduction of output; when there were 72 nymphs per ear, then the reduction in output was 19%; when there were 126 nymphs per ear, the reduction in output was about 8% (Han and Xu, 1982).
Chemical Control
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:
- EU pesticides database (http://ec.europa.eu/food/plant/pesticides/eu-pesticides-database/)
- PAN pesticide database (www.pesticideinfo.org)
- Your national pesticide guide
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