Conservation tillage crop yields in relation to grey garden slug [Deroceras reticulatum (Müller)] (Mollusca: Agriolimacidae) density during establishment

I

I ‘,

Conservation tillage crop yields in relation to grey garden slug [Deroceras reticulatum (Miiller)] (Mollusca: Agriolimacidae) density during establishment B. I. P. Barratt*, R. A. Byers and D. L. Bierlein USDA-ARS,

US Regional Pasture Research Laboratory,

University

Park, PA 16802, USA

Alfalfa, soybeans, maize, turnips and wheat crops established by conservation tillage, and that had been subjected to no slugs, medium or high densities of slugs Derocerus reticulaturn (Miillcr) at sowing, were harvested. The crop yields were inversely related to slug density, and seedling damage and mortality during crop establishment. Alfalfa and wheat suffered high levels of seedling mortality and their yields were reduced by up to 80%. Soybean yields were reduced by 2546% at the medium and high slug Maize grain yield was densities. which parallel plant survival after slug attack during establishment. reduced by -32% at medium and high slug densities in 1989, but drought conditions in 1088 precluded a grain harvest and there was no reduction in total plant yields where slugs were present at establishment. Turnips, which suffered only minor losses from slugs during establishment, consequently showed no yield reductions at maturity. Keywords:

Mollusca;

Deroceras reticulaturn;

Slugs (Mollusca: Pulmonata) are important pests of crops established by conservation tillage (Gregory and Musick, 1976). The absence of mechanical disturbance of the soil (Hunter, 1967) and retention of litter or residue (Dowling and Linscott, 1985; Hammond and Stinner, 1987) favour slug survival. They feed on seeds and seedlings of a wide range of crop species, including those important in the north-eastern United States such as alfalfa (Byers and Bierlein, 1984), maize (Hughes and Gaynor, 1984), soybeans (Hammond, 1985) and winter wheat (Gould, 1961). The relationship between grey garden slug, Deroceras reticulaturn (Miiller), density and pasture establishment has been studied in New Zealand in small plots that confined slugs at a range of densities (Ferguson and Barratt, 1983). A similar method using cages as enclosures for slugs was used to establish three density levels on a range of crops in Pennsylvania, USA (Barratt, Byers and Bierlein, 1989). This latter study showed that alfalfa (spring and autumn sown) and soybean establishment, and plant density, were successively reduced with increasing slug density. Maize, turnip and wheat establishment were less severely affected by slugs at the densities used, although sublethal damage was high for the latter two crops. Yields of these crops at maturity in relation to seedling losses

*To whom correspondence should be addressed, at lnvermay Agricultural Centre, Private Bag, Mosgiel, New Zealand 0261-2194/94/01/0049-04 @ 1994 Butterworth-Heinemann

no-till; crop yield

and sub-lethal in this paper.

Materials

injury

during

establishment

are reported

and methods

Field trials were located at Rock Springs Agricultural Research Center, Centre County, PA, USA on a Hublerstown silt loam soil (typic Hapludult; clayey, illitic, mesic). Average annual rainfall for the area for 1982-1988 was 1017 mm. Cages (800 X 1000 X 380 mm high) constructed from fine gauze polyethylene mesh attached to a wooden frame, open at the base and with removable lids, were used for the experiments. The cages were sunk 100 mm into the soil, and crops were sown inside the cages in such a way as to simulate conservation tillage sowing methods and at planting rates recommended for the area (Barratt et al., 1989). Planting dates, varieties and previous crop residues are shown in Table I. Maize planted in May 1989 was sown as above into ploughed soil rather than directly into the residue of the previous crop, as in all other cases. Locally collected grey garden slugs were introduced to the cages after sowing, at densities equivalent to 0, 10 and 20 mature slugs per cage (0, 12 and 24 mature slugs me2 or their equivalent). The slug developmental stages used were in proportion to those present in natural field populations at the time of collection. When only juvenile slugs were available, higher densities

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Conservation tillage crop yields and slug density: B.I.P. Barratt et al. were used to obtain what was judged, by size of the individuals, to provide similar pest pressure in all experiments. The slug densities established in the cages during these experiments were well within the range of those encountered commonly in conservation tillage field crops (Byers, Barratt and Calvin, 1989; Byers and Calvin, 1989). Slug survival during the experiments was not measured because this would have required destructive sampling of the caged plots. However, it was possible to view slugs in the refuge holes provided (five holes 25 mm X 150 mm deep between drill rows in each cage) and our observations suggested that survival was good, especially during the critical early stages of crop establishment. Bran baits containing methiocarb (2% a.i.) were used in the slug-free treatments, despite slug populations in the fields being low (< 1 m-‘). Regular inspection of refuge holes in these plots confirmed that no slugs were present. Each slug density treatment was replicated six times for each crop in a randomized block design. Because of

the low rainfall during summer 1988, cages were watered, supplying an equivalent of 12 mm to each cage up to three times a week during June and July. Cages were removed from the plots once the crops were established. Seedling damage, survival and plant densities were assessed during plant establishment and these results are reported in Barratt et al. (1989). Final established plant density data are given in Table 1. Harvest dates for all crops are shown in Table I. Alfalfa was harvested when -25 cm high, oven dried and weighed. Soybeans were harvested when the beans were fully mature, and the beans dried and weighed. Maize planted in June 1988 was severely affected by summer drought conditions and as it was not possible to measure grain yields, total plant yields were measured. Maize was cut to 4 cm above ground level and the whole plants were dried and weighed. In 1989, when the experiment was repeated, grain and ear weights were recorded. Turnips were harvested and bulb and leaf materials

Table 1. Mean crop yields at maturity in relation to slug density during seedling establishment

Crop

and cultivar

Previous residue

Slug density (no. m ‘)

crop Sowing

date

Harvest

date

I3 July

I988

Established crop density (no. m ‘)

Crop yield (g dry wt m “) whole plant

Alfalfa

‘WL 318’

Maize

19 April

I988

I6 August

Alfalfa

‘WL318’

4 August

Oat

1988

16 June

I988

I989

24 July 1989

Soybeans

Wheat

‘Pella’

‘Twain’

Maize ‘Doeblers

Maize ‘Doeblers

Maize

7 June

Oat

6 October

Maize

14 June

I988

I988

I989

I6 December

1988

21 July 1989

21 September

30 October

15 May 1989

None

1989

447 au 263 b 212 b

0 3’) 78

24 a 16 b 14 b

beans 466 a 346 ab 249 b

0 12 24

220 a I95 b 168 c

grain 172 a 78 b 20 c

0 16 32

11 a 9a Xb

whole 3475 3288 2650

12 a I2 a II a

grain yield 2074 a 1416 b 1406 b

ear weight I61 a 128 a 128 a

bulb 358 a 375 a 448 a

leaf 734 a 689 a 590 b

0

160 320

65X’

5 August

Oat

1988

I6 November

Crop Protection

1988

0

37.5 75

Star’

“Means followed by the same letter do not differ significantly 0, > 0.05)

50

lY88

58X

Turnip ‘Forage

1988

19 September

0 12 24 0 12 24 0 37.5 75 0 37.5 75 0 37.5 7s

1994 Volume

13 Number

1

524 a 329 b 254 c

113 a 99 b 8Y b

93 47 36 296 183 155 432 324 171 219 141 42 175 78 45

a b b a b b a ab b a ab b a b b

plant a a a

Conservation were dried and weighed separately. Wheat was harvested in July 1989 when mature and the dry weight of grain yield recorded. Yield data were analysed by ANOVA and means separated by Waller-Duncan K-ratio t test (p < 0.05) (SAS Institute Inc., 1982).

Results Alfalfa Alfalfa

yields were significantly reduced by slug presestablishment (Table I). In the springsown crop, there was no difference between yields from plots that had received the medium and high slug densities in parallel with plant densities at establishment. In the autumn-sown experiment, only the high slug density reduced yields for the first two cuts in the following season compared with the slug-free treatments. However, there was no difference between the medium and high slug density treatments. By September, however, yields were significantly reduced in both slug density treatments. ence during

Soybean Soybean yields were reduced only in the plots that had been exposed to the higher slug density during establishment. In these plots, bean yields were reduced by 46%) which relates fairly closely to the reduction of 42% in plant density in these plots at establishment. There was no reduction in yield per plant, suggesting that plants recovered from, or compensated for, sub-lethal injury caused by slugs, and that yield losses can be associated directly with plant mortality.

Wheat Despite slow germination and growth in cold autumn conditions, wheat seedling losses during the 30 days after sowing were only 12-20% in the medium and high slug density treatments respectively (Table I). However, during winter and spring, slug feeding continued and by mid-April 1989 seedling numbers were reduced by 46 and 70% in the medium and high slug density treatments, respectively. When harvested in July 1989, grain yields were significantly reduced by 54 and 88% at the medium and high slug densities, respectively.

Maize There were no reductions in plot or plant yields of maize in relation to slug density during establishment in 1988. However, in the 1989 crop, slugs at both medium and high densities resulted in significantly reduced total grain yields, despite plant survival remaining high for all treatments (Table I). Both medium and high slug densities resulted in a grain yield reduction of -32%,

tillage crop yields and slug density:

B.I.P. Barratt et al.

compared with yields from plots that were slug free. Ear weights were not reduced at either slug density.

Turnip Yields of the mature turnip crop were largely unaffected by the presence of slugs during establishment. Although plant numbers were reduced by -20% during establishment, final bulb weights were not reduced, suggesting that plant thinning by slugs allowed surviving bulbs to grow larger. Leaf dry weight was reduced by -20% only in the plots subjected to the highest slug feeding pressure in line with reduced plant numbers. Both the rapid growth of the turnip seedlings and the presence of oat seedlings in the plots, which had germinated from the previous crop, providing an alternative food supply for the slugs, may have contributed to the relatively minor effect of slugs on the final crop yields.

Discussion Slug damage and seedling mortality during establishment of conservation tillage crops can lead to substantial yield losses. Turnips were able to compensate for early seedling damage in this trial with little effect on yields. Other crops such as alfalfa and wheat, which suffered high levels of seedling mortality, yielded reduced levels of dry matter roughly in proportion to plant density. Despite seedling establishment being higher in the autumn-sown than in the spring-sown alfalfa, sub-lethal injury levels were higher in autumn and this may have led to plant mortality over winter accounting for the greater effect on yield from slug presence during autumn compared with spring-sown alfalfa. Maize in 1988 was too severely affected by the drought during summer for slug predation to have affected yields, but in the more favourable climatic conditions of 1989, grain yield was reduced by >30% in the plots that were exposed to slugs. As the plant density at establishment was not significantly reduced, sub-lethal effects from slugs at this early stage were thought to be responsible for grain yield reductions. Alfalfa, soybeans, maize (in 1989) and wheat yields were reduced in crops that suffered slug damage during establishment but, with the exception of the latter, this was irrespective of the slug densities tested. This supports the findings of Ferguson and Barratt (1983), who found a similar non-linear relationship between slug density and plant damage where white clover seedling mortality per slug was reduced with increasing slug density. In the case of wheat, where seedling growth was slow in the cool autumn/winter conditions and slug feeding activity would have been limited by low temperature, the apparent density dependence may become less important with feeding occurring at a lower intensity but over a longer period. Sub-lethal damage may -also be a more-important factor for overwintering plants such as alfalfa, reducing the capacity for plants to recover and compensate.

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Conservation tillage crop yields and slug density: B.I.P. Barratt et al.

Acknowledgements

We would like to thank Caroline Bahler, Pat Higgins, Charlie Biddle, Joel Ake, Sean Erwin and Aaron Barto for assistance in the field.

Dowling, limitation 394-402

P. M. and Linscott, D. I,. (1985) Slugs as a primary to establishment of sod-seeded lucerne. Crop Prot. 4,

Ferguson, C. M. and Barratt, B. I. P. (1983) Slug damage to pasture renovated by direct drilling. Proc. 36th N. Z. Weed Pest Control Conf. 2 12-2 I 5 Gould, M. J. (1961) Observations on slug damage to winter wheat in East Anglia. 1957-1959. Plant Pathol. 10, 142-146

References Barratt, B. I. P., Byers, R. A. and Bierlein, D. L. (1989) Conservation tillage crop establishment in relation to density of the field slug (Deroceras reticulaturn (Muller) ). In: Slugs and Snails in World Agriculture, BCPC Monogr. 41 (Ed. by 1. F. Henderson) pp. 93-99, BCPC, Thornton Heath Byers, R. A., Barratt, B. 1. P. and Calvin, D. (1989) Comparison between defined-area traps and refuge traps for sampling slugs in conservation tillage crop environments. In: Slugs and Snails in World pp. 187-192, Agriculture, BCPC Monogr. 41 (Ed. by I. F. Henderson) BCPC, Thornton Heath Byers, R. A. and Bierlein, D. I,. (1984) Continuous alfalfa invertebratc pests during establishment. J. Econ. Entomol. 77, 1500-1503 Byers, R. A. and Calvin, D. (1989) Reduction in corn survival, growth, and yield caused by juvenile slugs. Proc. 26th Northeast Regional Field Crops Insect Conf. (Ed. by A. A. Hower, Jr) pp. 47-52, unpublished report

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Gregory, W. W. and Musick, G. J. (1976) Insect management reduced tillagc systems. Bull. Entomol. Sot. Am. 22, 302-304 Hammond, R. B. (1985) Slugs as a new pest of soybeans. Entomol. Sot. 58, 364-366 Hammond, R. B. and Stinner, B. R. (1987) Seed corn (Diptera: Anthomyiidae) and slugs in conservation systems J. Econ. Entomol. 80, 680-684

in

J. Kansas maggots in Ohio.

Hughes, K. A. and Gaynor, D. L. (1984) Comparison of Argentine stem weevil and slug damage in maize direct-drilled into pasture or following oats. N. Z. J. Expl Agric. 12, 47-53 Hunter, P. J. (1967) The effect of cultivation Plant Pathol. 16, 153-156

on slugs of arable

land.

SAS Institute Inc. (1982) SAS User’s Guide: Statistics. SAS Institute, Cary, NC Received 27 March 1992 Revised 17 May 1993 Accepted I7 May 1993