E L S E V I E R Agriculture, Ecosystems and Environment 52 (1995) 51-55

Agriculture Ecosystems & Environment

Population dynamics of aphids and syrphid larvae in winter wheat treated with different rates of pirimicarb *

Berthold Niehoff*, Hans-Michael Poehling 1 University of GOttingen, Institute for Plant Pathology, Department of Entomology, Grisebachstrasse 6, D-37077 Giittingen, Germany

Abstract

The effects of different rates of pirimicarb on aphids and syrphid larvae in winter wheat were studied from 1991 to 1993. The numbers of aphids and pre-imaginal stages of aphid-specific predators were determined visually in four replicated randomised plots.

The years showed strong differences in aphid infestation, syrphid migration and oviposition, and larval density probably because of different weather conditions. The results indicate high efficacy of pirimicarb to aphids but also to syrphid larvae at the currently recommended rate ( 100 g a.i. ha- 1 ) and even at 50 g a.i. ha- 1. A certain degree of selectivity was obtained at 25 g a.i. ha- i.

Keywords: Aphids, cereal; Selectivity; Syrphids; Winter wheat

1. Introduction

Cereal aphids have been the main pest of winter wheat during the last several years. As a result, regular treatments of large areas are necessary to prevent yield losses. As economic thresholds are low, frequent appli- cations are needed which contradict the use of natural enemies. A strategy has to be developed to include optimal timing and reduction of side effects of insec- ticides to non-target organisms, particularly beneficial arthropods like aphid antagonists. One way to increase selectivity of common pesticides might be to reduce rates and adapt them more carefully to aphid population

* Corresponding author. Present address: University of Hannover, Institute for Plant Pathol-

ogy and Plant Protection, Herrenh~iuser Strasse 2, D 30419 Hanno- vet, Germany.

This project was supported by the German Ministry of Agriculture.

0167-8809/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDIO167-8809(94)O9009-V

dynamics. Some selective effects may be obtained from the steeper dose-response curves of many predators compared with their prey (Van Emden, 1988 ) and from maintaining a residual pest population to avoid star- vation of natural enemies (Poehling, 1988). Reduced rates can also give an improved economic return (Mann et al., 1991).

In most years syrphid larvae are very efficient pred- ators in the area of G6ttingen, Germany (Tenhumberg, 1992). The present project investigated the possibilities and limits of cereal aphid control using low rates of pir- imicarb insecticide.

2. Material and methods

The data were derived from three field experiments, made from 1991 to 1993 at a location near G6ttingen. In all years four replicated randomised plots of winter

52 B. Niehoff H.-M. Poehling /Agriculture, Ecosystems and Environment 52 (1995)51-55

wheat were set up. Plot size was 660 m 2 (12 m×55 m) in 1991 and 1320 m 2 (24 m × 5 5 m) in 1992 and 1993. Pirimicarb ( 'pirimor' ) was sprayed at 25, 50 and 100 g a.i. in 400 1 ha-~. Insecticides were applied at EC 69 (Zadoks et al., 1974) in 1991 and 1992 and at EC 71 in 1993, leaving the control plots untreated. Aphid densities and pre-imaginal stages of aphid-spe- cific predators (syrphids, coccinellids and chrysopids) were recorded continuously from the beginning of June until the breakdown of the aphid population, based on samples of 160 shoots (4 × 40) per plot; shoots were cut off at the bottom and the aphids and predators counted immediately in the field.

3 . R e s u l t s

3.1. 1991

The development of the cereal aphid population started at the beginning of June (EC 51 ). After a period of rather humid and cold weather in May and especially in the first half of June, the aphid population increased until the end of flowering to an average of 12 aphids per shoot (Fig. l ) . Spraying was performed at a thresh- old of five aphids per ear and flagleaf at the end of flowering (EC 69, 5 July). Aphids reached a peak of 16.6 per shoot and collapsed soon afterwards. Predators were nearly exclusively syrphid larvae (99%), chry- sopid and coccinellid larvae were not found.

The first syrphid eggs were found at the end of June (EC 61), and larvae appeared at the beginning of July and reached a very high density for 6 days (more than 0.2 larvae per shoot). After the aphid population col- lapsed, only a few larvae could be observed (Fig. 1). Pupae were found 7 days after the first larvae and were counted during a 3 week period (8 July-29 July), reaching a peak of 0.1 per shoot. A relationship between rate and effects on syrphid larvae was detected (Table 1). Fewer larvae were discovered in all plots after treat- ment. At 25 g a.i. ha- t, larvae reached a maximum of 0.12 per shoot but very few pupae could be found. Similar numbers of larvae and pupae were found in plots sprayed with 50 and 100 g a.i. ha- ~.

3.2. 1 9 9 2

The development of cereal aphid population started at the beginning of June (EC 55). The population

0,8 ~ t r e a t e d ] - 20

-15 -8 -4 -1 +1 +3 +6 +10 +13 +17 +20 +24 +31

0,8 [2.~ + ~+i aa- 157 . . . . . . 20

°'+ /%, '1 ,o

oi I : ! , -15 -8 -4 -1 +1 +3 +6 +10 +13 +17 *20 +24 +31

0,8 . . . . 20 ~. 50 g a L / h a [

;+ 1,+! 0.4+ /1', +! -lo -

0 " i 2. _

-15 -8 -4 +1+1+3 +6 +10+13 +17+20 +24 +31

0.+ ' 100~,?, ~, 7- . . . . . . . . . . . . . . T2°

0,+4

-15 -8 -4 -1 +1 +3 +6 +10 "-13 +17 +20 +24 +31 days before or after t rea tment

EC: 55 61 69 71 75 79 85 91

[ - ~ eggs ~ larvae I pnpae _ aphid_s j

Fig. 1. Abundance of aphids and pre-imaginal stages of syrphids in winter wheat plots treated with different rates of pirimicarb (Gtittin- gen, 1991 ).

increased very quickly for 24 days to a peak of 170 aphids per shoot at EC 75 (Fig. 2). The population collapsed during the following 2 weeks.

Treatments were performed on 20 June at the end of flowering, at an average density of 20-25 aphids per shoot, or 7-10 aphids per ear and flagleaf.

As in 1991 predators comprised mainly syrphids (63 %), but with higher percentages of coccinellid and chrysopid larvae (24% and 13% respectively).

Syrphids started oviposition at EC 65, as already more than ten aphids per shoot (Fig. 2) had established. In the control plots oviposition continued until EC 79 (mid-July), when the aphid population collapsed. The first larvae occurred at the end of flowering and the density oscillated until the end of the investigation. A

B. Niehoffl H.-M. Poehling /Agriculture, Ecosystems and Environment 52 (1995)51-55

Table 1 Average densities of syrphid larvae per shoot of winter wheat in plots treated with different rates of pirimicarb in 1991

53

Days after treatment

- 1 1 3 6 10 13

Untreated 0.04a 0.23a 0.22a 0.26a 0.05a 0.03ab 25 g a.i. h a - ~ 0.07a 0.1 lab 0.12b 0.07ab 0.03a 0.05a 50 g a.i. h a - ~ 0.09a 0.06b 0.01c 0.04b 0.04a 0.01b

100 g a.i. ha - ~ 0.08a 0.03b 0.03bc 0.01 b 0.02a 0.03ab

Means followed by the same letter are not different (P_< 0.05, Kruskall-Wallis test by Wilcoxon scores).

peak of larval abundance was detected at the end of July in the treated plots. The pupal stage was reached 12 days after the first larvae were found.

In the treated areas a second oviposition peak occurred in relation to a second build-up of aphids 1

6 T . . . . 180

5 ~' -~ + 150

= , ~ !120 3 • ",, ~90 2

2 - ~', ~60 ~ O °,, -5 0+1 +3 +5 +7 +9 +12 +16 +19 +23 +26 +30

6 . . . . 2 5 g a i / h a I 83 - -~ Ig0

~ 4 ! ~ 1 2 0 ~

£ _ - - _ I ~ I 60

-5 0+1 +3 +5 +7 +9 +12 -'16 +19 +23 +26 +30

6 r 1 . . . . . . ] 180 5 '50 g ax /h a VI 206 t _ 1 5 0

-5 0+1 +3 +5 +7 +9 +12 +16 +19 +23 +26 +30

6 . . . . . 180 100 g a.i./ha 92 ,

-5 0+1 +3 +5 +7 +9 +12 +16 +19 +23 +26 +30 days before or after treatment

E C 65 69 71 75 79 85 91

t! ! 7~_ eggs . . . . . . . ~ larvae I pupae - - a p h i d s ~

Fig. 2. Abundance of aphids and pre-imaginat stages of syrphids in winter wheat plots treated with different rates of pirimicarb (G6ttin- gen, 1992).

week after treatment, the total numbers of aphids remaining lower than in control plots. The trial in 1992 showed similar trends to 1991, although there were marked differences in aphid and syrphid densities between these years. In contrast to 1991, no significant differences in larval density could be detected.

Pirimicarb caused high initial toxicity to aphids regardless of the exposure at the ear or the flagleaf. Larval densities differed: at the lowest insecticide rate larvae were found continuously but there were no lar- vae at the medium dosage for 1 week, or at the rec- ommended rate for more than 2 weeks.

3.3.1993

Aphid infestation started at the beginning of June (EC 55). There was only a small increase in the aphid population to a peak of 9.7 aphids per shoot on 12 July (EC 79), followed by a population collapse during the following 2 weeks (Fig. 3).

Treatments were applied on 21 June (EC 71) at an average density of 2 aphids per shoot. As in the previous years predators mainly comprised syrphid larvae (87%), followed by coccineUid (12%) and chrysopid larvae (1%).

Syrphid eggs were found from the beginning of June until the end of the investigation, and larvae were always present with a peak of 0.1 larvae per shoot on 8 July when the aphid population was still increasing. The first pupae were noticed 8 days after the first larvae occurred, peaking at 0.03 per shoot on 8 July at the highest larval density (Fig. 3).

The treated plots showed a relationship between dos- age and larval density of syrphids: at 25 g a.i. ha- ~, the numbers of larvae were higher than at the medium and recommended rate (Table 2).

54 B. Nieholf H.-M. Poehling /Agriculture, Ecosystems and Environment 52 (1995) 51-55

Table 2 Average densities of syrphid larvae per 100 shoots of winter wheat in plots treated with different rates of pirimicarb in 1993

Days after treatment

0 1 3 5 7 10 14 17 21 24 30

Untreated 0.6a 1.3a 3. la 1.3a 1.3a 2.5a 5.0a 10.6a 8. la 3,8a 3.1a 25 g a.i, ha- ~ 1.9a 1.3a 0.0a 0.0a 1.3a 1.9a 1.3a 1.3ab 5.6ab 2,5a 1.9a 50 g a.i, ha- ~ 3. l a 0.6a 0.0a 0.6a 0.0a 0.6a 0.6a 0.6ab 1.9ab 0,6a 3.8a

100 g a.i. ha- ~ 1.9a 1.3a 0.0a 0.6a 1.3a 1.3a 1.3a 0.0b 0.0b 1.9a 1.3a

Means followed by the same letter are not different (P_< 0.05, Kruskall-Wallis test by Wilcoxon scores).

211 110 . . . . . . . ,

m ' 12- J

- l l -7 -3 0+1+3+5 +7 +10 +14 +17 +21 +24 +30 +35

20 Ill 25 g a i /ha

~ 12! ,~'~' ~ ~ ~

I ~i . . -i~ . 8

- I ! -7 -3 0 + 1 + 3 + 5 + 7 +10 +14 ~17 +21 +24 +30 +35

2 0 . . . . ; 0 ! 50 g a i/]la

16 i 8 >

12 ~6 V

-11 -7 -3 0+1+3"~5+7 +tO +14 +17 +21 +24 +30 +35

20 '100 g a . i '% 7 cn [ - - 1(1

2 h2: ! 6 v

m i, r ! t

-11 -7 -3 0 + 1 + 3 + 5 + 7 +10 +14+17 +21 +24 +30 +35 days before or aider t rea tment

EC: 65 69 71 75 79 85 91

21 eggs ~ ' ~ larvae m pupae - - aphids 7

Fig. 3. Abundance of aphids and pre-imaginal stages of syrphids in winter wheat plots treated with different rates of pirimicarb (G6ttin- gen, 1993).

4 . D i s c u s s i o n

Syrphid larvae were the most abundant aphid pred- ators in the present investigations, this is in correspon- dence with other studies (Chambers and Adams, 1986; Tenhumberg, 1992; Groeger, 1993).

The reasons for different oviposition rates and larval densities in 1991 and 1992 are not clear and may be the result of different weather conditions (cool and humid in 1991 compared with warm and dry in 1992). Despite early wheat development and a strong increase of aphids in 1992, syrphids did not react to increasing aphid density as in 1991. The egg-laying behaviour of syrphids was perhaps influenced by temperature (Ankersmith et al., 1986) or humidity (Wahbi, 1967) in 1992. In 1992 the first eggs and larvae were probably produced by the overwintering syrphid population, whereas the second peak might have been due to migrating syrphids. Long distance migration is regarded as very important for the population build-up of syrphids in northern areas during the summer (see Tenhumberg and Poehling, 1995) and depends on weather conditions. Larvae occurring at the end of July were unlikely to complete development. According to Ankersmith et al. (1986) only one generation is able to complete development on cereals under the climatic conditions of the Netherlands.

Aphid populations on wheat may be perfectly con- trolled by pirimicarb at rates lower than those currently recommended. In all years there was a relationship between the density of syrphid larvae and the applied rate of pirimicarb. The results indicate that there was no residual population of prey to avoid starvation at high pirimicarb dosage but 25 g a.i. ha- l allowed both aphids and syrphid larvae to survive treatment in all years (Tables 1 and 2, Figs. 1, 2 and 3).

The toxicity of pirimicarb to syrphid larvae is known from laboratory investigations (Graepel, 1981; Brown et al., 1983; Storck-Weihermiiller, 1987). Field studies also confirm its toxicity to aphids and syrphid larvae when highly dosed, but there is a possibility of enhanc- ing the selectivity of pirimicarb to these predators by

B. Nieh~ff~ H.-M. Poehling /Agriculture, Ecosystems and Environment 52 (1995) 51-55 55

spraying reduced dosages (Poehling and Dehne, 1986; Poehling, 1988). The dose-response curves of preda- tors is often steeper than these for the pest (Van Emden, 1988). As a result, the mortality of the predator decreases faster than that of the pest as dosage is reduced. The present investigation shows that in agree- ment with other investigations (reviewed by Hull and Beers (1985) and Poehling (1989)), dose reduction may supplement other techniques to increase the selec- tivity of pesticide use.

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