Neth. J. PI. Path. 99 (1993) Supplement 3 : 5 1 - 5 6
Surveys of cereal diseases and pests in the Netherlands. 6. Occurrence of insect pests in winter wheat R.A. DAAMEN j and W. STOL 2. DLO Research Institute for Plant Protection (IPO-DLO), P.O. Box 9060, 6700 GW Wageningen, the Netherlands 2 Netherlands Grain Centre (NGC), Costerweg 5, 6702 AA Wageningen, the Netherlands Accepted 13 September 1993
Abstract Between 1980 and 1986, commercial winter wheat fields at the milky-ripe stage were surveyed for insect pests. Leaf injuring insects were common. On average 42% and 95% of the surveyed fields were injured by leaf miners and the cereal leaf beetle, respectively. On average 14% of the leaves was skeletonised by the cereal leaf beetle and leaf injury tended to be high in seasons after winters with low temperatures. Blossom midges were the most important midges. In the period 1980-1984, the orange and lemon blossom midge infested 71 and 21% of the surveyed fields, respectively. Insecticides were mainly used to control aphids. Despite control, on average 80% of the fields and 22% of the tillers were infested by aphids.
Additional keywords: Triticum aestivum, Oulema melanopus, Mayetiola destructor, Haplodiplosis marginata, Sitodiplosis mosellana, Contarinia tritici. Introduction Systematic annual surveys of diseases and pests in commercial winter wheat fields in the Netherlands were conducted during the years 1974-1986. In previous papers, the epidemics of fungal diseases has been reported. F r o m 1980 onwards, infestations by insects were recorded during the surveys in July. In this p a p e r a compilation is made of the annual survey reports (Daamen et al., 1981, 1982; Daamen and Wietsma, 1983, 1984; Stol, 1985; Versluis, 1985; Van den Hoek, 1986) to describe the occurrence of insect pests in winter wheat at the milky-ripe stage and to relate their intensities to weather and cultivar resistance. This paper is based on observational data. Hence, a description is given of the pest intensities in commercial fields during 1981-1986, and, some factors which may affect these intensities are indicated. Materials and methods Cropping practices and field selection have been described in the first paper in this series (Daamen, 1990). In each survey about 100 commercial winter wheat fields were inspected (Table 1). In the commercial fields insecticides, mainly Pirimicarb and Dimethoate (Daamen, 1990) were used to control aphids (Table 1). The intensity of other insects will * Present address: DLO Centre for Agrobiological Research (CABO-DLO), P.O. Box 14, 6700 AA Wageningen, the Netherlands. 51
Table 1. Prevalence (% fields infested) and mean incidence (% leaves or % tillers infested or damaged) by insect pests in winter wheat between 1980-1986. Year
1980
1 9 8 1 1 9 8 2 1 9 8 3 1 9 8 4 1 9 8 5 1986
Number of fields surveyed Leaf miners: prevalence incidence, leaves
164 30 _1
138 47 1.4
152 40 0.9
143 36 0.8
123 49 1.1
94 41 0.1
94 51 1.3
80 -
93 10.7
99 17.1
97 13.4
95 9.5
99 16.4
100 16.3
Hessian fly: prevalence
0
2
1
1
0
0
7
Saddle gall midge: prevalence
0
0
1
0
0
0
0
55 16.6
82 24.7
89 28.0
85 1 8.7
91 27.3
-
76 17.3
74 5 0.0
58 43 0.1
105 15 0.0
95 20 0.0
-
-
51 0.0
70 0.1
95 0.8
82 0.5
55 0.1
-
-
70
84
95
80
Cereal leaf beetle: prevalence incidence, leaves
Aphids: prevalence incidence, culms Number of fields sampled Lemon blossom midge: prevalence larvae/ear Orange blossom midge: prevalence larvae/ear Percentage of fields treated with insecticide against aphids 1 --
=
no
106 -
101
observation.
have been affected too, but as the types and the frequence of the used pesticides was rather stable, it is presumably not the main cause of variation in pest intensity between the years. It was not possible to assess each year the peak intensity for each pest. Instead, intensity of pests was assessed in samples of 40-50 tillers per field during July, at the milkyripe stage (Daamen and Stol, 1990). To quantify pest intensity, the annual pest prevalence (the percentage samples infested) was determined. From 1981 onwards, pest incidence (percentage tillers or leaves infested) was also determined in each sample, to obtain more detailed information. For the leaf injuring insects (cereal leaf beetle and leaf miners) the number of injured leaves was counted and expressed as percentage of the total number of leaves in the sample. For aphids and the Hessian fly, the number of tillers infested was counted and expressed as percentage of total tiller number per sample. Incidences were averaged over all fields to obtain an estimate of mean annual incidence.
Blossom midges.
Between 1980 and 1984, the number of larvae of blossom midges per ear was assessed. In part of the fields (Table 1), ears were cut of the tiller samples. In the laboratory, the ears were placed in boxes with a mesh screen as bottom, above a black box filled with one cm. water. Ears were wetted three times a week to stimulate larvae to leave the ears. Larvae, collected in the water, were counted twice a week during about 3 months.
Other insects. Deadhearts caused by Delia coarctata and Oscinella frit occur occasionally in the Netherlands, mainly in the northeastern part of the country. Injury caused by these insects could not be assessed during the July surveys. Occurrence of thrips and additional insects was not assessed. 52
Neth. J. PI. Path. 99 (1993) Suppl. 3
Correlations. Correlation coefficients between annual pest intensities and average monthly temperatures (~ total monthly precipitation (mm) and average monthly sunshine duration (expressed as percentage of total daylength) during the cropping period were computed, to explore associations of pest intensities and weather. The average occurrence of pests in each cultivar was described in the annual survey reports. Here it is reported when cultivars show consistent effects on pest intensities during the survey period. It is stressed that obtained relations between the observational data are correlative. Results and discussion
Leafminers.
Species of leaf mining insects were not identified. Small narrow mines at random positions on the leaf were most common, while big wide mines starting from the top of the leaf, presumably caused by Hedrellia griseola, were rare. In the years 1980-1986 on average 42% of the fields surveyed was injured by leaf miners. On average 0.9% of the leaves was mined (Table l ). Both measures of pest intensity were positively but not significantly correlated (r = 0.6). Annual intensities o f leaf miners did not show any significant and consistent correlations with monthly weather characteristics.
Cereal leaf beetle. In the Netherlands, Oulema melanopus dominates over O.lichenis. Between 1980 and 1986, on average 95% of the fields was injured by larvae of the cereal leaf beetle. On average 14% of the leaves was skeletonised by the larvae (Table 1). Both measures of pest intensity were positively correlated (r = 0.9). During these years, cereal leaf beetle intensity did not increase with time as observed in Germany (Heyer and Wetzel, 1990). This increase was presumably caused by increased crop densities (Shade et al., 1970; Honek, 1990), which did not increase much after 1980 in the Netherlands. Annual prevalence and mean annual incidence of injury by cereal leaf beetles tended to be negatively correlated with average temperature over the months December, January and February (r = -0.6 and r = -0.8 for prevalence and incidence, respectively). This agrees with the results of Ali et al. (1979) of sharply increasing mortalities of overwintering adult beetles at temperatures above 10 ~ Incidence of the cereal leaf beetle in the winter wheat cultivar Okapi was consistentty lower than that of the other common cultivars (Table 2). This indicates differences in
Table 2. Mean incidence (% leaves injured) of the cereal leaf beetle in different winter wheat cultivars during 1981-1986, July surveys. Number of fields surveyed is given in brackets; only cultivars with more than 10 observations are tabulated. Cultivar
Nautica Arminda Okapi Marksman Saiga Citadel Granada Obelisk
Year 1981
1982
19 12 8 9 .
. 20 ll 21 21 -
(13) (52) (55) (18)
.
Neth. J. Pl. Path. 99 (1993) Suppl. 3
1983 .
(45) (58) (22) (24) .
12 10 19 23 10 .
1984
. (41) (49) (15) (21) (17)
. 11 6 14 10 11 .
1985
1986
26 (43) 13 (25) -
20 (27) 10 (19) 24 (20)
. (37) (43) (16) (13) (14)
53
resistance among cultivars, and the possibility of resistance breeding to control epidemics of cereal leaf beetles.
Hessianfly. Prevalence of the 'flaxseed' stage of the Hessian fly (Mayetiola destructor) averaged 1.6% of the fields between the years 1980 and 1986 (Table 1). Saddle gall midge. The saddle gall midge (Haplodiplosis marginata) is rare in the Netherlands. It is locally observed, mainly in the northeastern part of the country where cereals are cropped intensively on heavy sea clay, confirming observations in the United Kingdom (Gair et al., 1972) and Germany (Weidner, 1985; Basedow, 1986). During the surveys, galls were found in the sample of one field in this region in 1982 only. Blossom midges. Between 1980 and 1984, the orange blossom midge (Sitodiplosis mosellana) was more common than the lemon blossom midge (Contarinia tritici) (Table 1). Prevalence averaged 71 and 21% of the ear samples for the orange and the lemon blossom midge, respectively. Pest density averaged 0.3 larvae per ear for the orange blossom midge, but was below 0.05 larvae per ear for the lemon blossom midge. Annual prevalence and average annual density of the orange blossom midge were positively correlated (r = 0.9). In Germany (Basedow and Schutte, 1982), Switzerland (Affolter, 1990) and Finland (Kurppa, 1989) both species were about equal in importance. In the Netherlands during 1968 and 1969, Sitodiplosis mosellana was slightly more dominant than Contarinia tritici (Nijveldt and Bokhorst, 1973). However, in the present study the intensity of C. tritici (Table 1) may have been underestimated due to sampling in the second half of July, as the lemon blossom midge infests wheat earlier than the orange blossom midge (Barnes, 1956). Intensities of both blossom midges increased during 1980-1982 and declined during 1982-1984 (Table 1). This could have been due to weather conditions, as intensities of the blossom midges were positively correlated with average temperature during June and negatively with precipitation during July (Table 3). These characteristics combined explained statistically the main part of the annual variation in prevalence and density of the orange blossom midge infestations (R 0.9), but not of the lemon blossom m i d g e (R 2 = 0.3). High temperatures during June may stimulate flights of the midges and oviposition. The negative correlation with precipitation du¡ July could indicate that sampling was on average too late to assess peak midge intensity, as full-grown larvae migrate from the ear to the soil during periods with high precipitation (Barnes, 1956; Basedow and Schutte, 1982). Aphids. In winter wheat Sitobion avenae usually predominates and Metopolophium dirhodum is often co-dominant while Rhopalosiphumpadi is common. Between 1980 and 1986 (1985 missing) at the milky-ripe, on average 80% of the fields and 22% of the tillers Table 3. Correlation coefficients between annual intensities of the lemon and orange blossom midges and average temperature in June and total precipitation in July. Weather condition June temperature July precipitation 54
Lemon prevalence 0.56 -0.56
Orange prevalence 0.85 -0.88
Orange larvae/ear 0.88 -0.80
Neth. J. PI. Path, 99 (1993) Suppl. 3
was infested by aphids (Table 1). Both assessments of aphid intensity were positively correlated (r = 0.75). No significant and consistent correlations were found between annual monthly weather variables and aphid intensity. However, it should be realised that aphid intensities were assessed in commercial fields where insecticides were commonly used to control outbreaks (Table 1).
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