T'HE EFFECT' OF MOWING SPRING WEED HOST'S OF HELfOTHIS SPP. ON PREDATORY ARTHROPODS!.'

Vivienne E. Harris and Jacob R. Phillips Department of Entomology

University of Arkansas Fllyetteville. AR 7270 l

Aostrfli"t: Regular mowing of spring weeds growing along roadsides and in other non-field areas in n 65 km 2 area reduced predatol')' anthropod population density in the weeds by 1i~;' in the firsl year and 32% in the second, compared with a similar size unmowed area. The effect on the different predator groups was not unifonn. Oritls spp. were unaffected and spiders slighdy affected, while Geocoris spp.. lady beetles. and lacewings were reduced in density by 45·50%. Overall prednl.or density was reduced more severely in crimson clover (7'ri{o!ium ir/(;lImatum Linnaeus) than in other weed planL.. sampled.

Key \Vords: Predators, DrillS. Gcocori.'i. lady beetles, lacewings, nubids, spiders, wild host plants, crimson clover, vetch, gernnium, Heliutllis, mowing.

J. Agric. Entomol. 3(1): 77-86 (January 1986)

1\\'0 insect species, HeliotJll~~' zea Boddie and H. uirescens (Fabricius), comprise one of the most destructive and ecologically disruptive pest problems in the United States (Knipling and Stadelbacher 1983). In the Mississippi Delta area of Arkansas and Mississippi, t.he average annual losses due to crop damage and costs of cant rol for these two species were estimated by Knipling and Stadelbacher (l983) at $35 million per state. Recently much attention has been given to the possibility of severely reducing the population levels of these insect species in the first generation (Gd, as a means of retarding population build-up and thereby maintnining their densities at acceptable levels in crops in generations G3 , G.l • and G;j. The theory is based on the following: (1) Heliothis numbers are at their lowest in the spring (Gd after heavy wimer mortality (Stadelbacher L982); (2) because of the absence of suitable cultivated crops in the spring, overwintered moths, whether locally emerged or immigrant, oviposit on wild host plants (here referred to as "weeds"), which serves to concentrate the G1 eggs and larvae in the relatively small acreage covered by these weeds 13.5% of the rural area (Stadelbacher 1982)1; and (3) when resultant moths emerge from the weeds they oviposit on seedling crops covering an acreage 14 times that of the weeds (est.imates for 1\'lississippi Delta. Knipling and Stadelbacher 1983). thus greatly dilllling the density of G2 eggs and larvae relative to GI , and greatly decreasing their accessibility too natural enemies. The potential benefits of directing control tact.ics at G] in the weeds are obvious.

Appropriat.ely timed mowing of weeds, herbicide application, insecticide application, lise of Heliothis-specific microbial agents, alltocidal control, and release of HeJiothis parasites have been the tactics most often suggested for reducing G1 Heliothis population levels (Knipling 1979; Stadelbacher 1982;

Puhlished with approval of Director, ..\rknni'llS Agricultural Experiment Station. Received for publication 2; ,lImuftl')' 1984: accepted; April 1986.

2 This matl'rial is based upon the work llupportl'd by USDA under Agreement No. 82~CRSR-2·1000 and by Ben J. Altheimer Foundation, Ahheimer. AR.

77

I

78 J. Agric. Entomol. VoL 3, l\o. I (1986)

Knipling and Stadelbachcl' 1983; Mueller ct al. 1984). Each tactic has advantages and disadvantages in terms of estimated cost-effectiveness and impact on other arthropods coinhabiting the weeds. The first three. in particular, evoke the question: What will be the impact on predatory arthropods in the weeds, which in most cases are species lat.er found in the same crops as Heliolhis? The research reported here addressed this question in relation to mowing wayside weeds over a large area.

MATERIALS AND METHODS

The study was performed in the spring of 1982 and 1983 in western Desha Co. and northeastern Drew Co. of southeastern Arkansas. Two 65 km 2 areas ,,\-'ere compared for predatory arthropod density in weeds growing along roadsides. railroad rights-of-way, turnrows, field edges, ditch and other watenvay banks. These habitats accounted for ca. 2.3 km 2 (230 hal in each area (3.5% of 65 km ~,

see Stadelbachcr 1982). In one of these areas (Pickens, a single large farm) mowing of all weedy non-field areas is a routine prl1ctice. \Ve had no control over when or where it occurred, but observed that. mO\"'ing began in early April and that. any given area would be remowed every 2 . 3 wk. Because of the large size of the farm, all weedy areas were not mowed on the same day 01' same week. Mowers cut. the plants off ca. 13 em from the ground. A 65 km 2 mult.i-farm area to the south of Pickens seIY'ed as an unmowed comparison. At their closest points, t.he mowed and unmowed areas were separated by 3.2 km. Topography, weed species, and crops planted (mainly soybean, cotton, and rice) are similar in the two areas.

The mowers in Pickens did nOt always pass over every plant potentially harboring predatory arthropods. Narrow strips of weeds were sometimes missed in an otherwise mowed stretch. For example, adjacent. passes by a mower on a wide right-of-way did not always abut perfectly. The difficulty of maneuvering a tractor on steep slopes or close to utility poles and other obst.acles also hampered total coverage by the mowers. Such unmowed or sometimes mowed weeds were nevertheless sampled, because the intent. was to compare predator population levels in wayside dicotyledonous weeds in two large areas - one where such weeds were mowed regularly as part of routine on-the-farm operat.ions, and the other where they were not mowed.

Sampling proceeded [rom 29 March to 3 July 1982 (14 wk) and [rom 11 April to 16 June L983 (10 wk) Cll. 3 d/wk. In general, a day of sampling in the mowed area was alternated with a day in the unmowed m·ea. A sample unit was 10 sweeps (I sweep-set) with a sweep-net having a 38-cm diameter opening. Only dicotyledonous, spring-flowering weed species were sampled (i.e. known or possible Helinthi... hosts), and all such species encountered in the study area were sampled, provided they occurred in large enough clusters to be covered by at least 10 sweeps at a site. As the clicots were most commonly growing in single-species clusters, it was a relatively simple matter to sample the various species separately. The occurrence of a few grass plants in a single-species dicot. stand was often unavoidable. A category "rni.xed weeds" was designated to cover those Ilatches where four or more dieot species grew closely intermingled.

Sites for sweeping were chosen in the following manner. Each week, all paved and unpaved roads in the two areas, as well as many rarm turnrows, were traversed slowly by truck. When a dicot stand, which had not been sampled

79 HAHRIS and PHILLIPS: Effect of Mowing \\leeds on Predators

previously in the week, was sighted, the truck was stopped and the stand sampled. The number of sweep-selS taken per stand depended on the size of the stand and varied from 1 La 20. A newly mowed stretch of weeds was not resampled until ca. 1 wk after mowing.

The various weed species were not equally common in the study region and the sample size (number of sweep-sets) per weed type renectcd these differences, being approximately proport.ional to their respective abundances in the region. As our principal aim was to compare two areas (mowed vs. unmowed), with comparison among weed types only a secondary aim, we felt. that unequal sample sizes among weed types was acceptable. However, sample sizes in the mowed and unmowed areas were approximately equal for any part.icular weed within anyone week. In May the average number of sweep· sets taken in each of the two areas was 55/wkJarea Fewer sweep-set.s (37/wk/area) were taken in April because many of the weed species had not. yet appeared. Similarly, fewer sweep-sets were taken in June (28/wklarea) because many of the species had died back.

Predat.ory arthropods were identified and tallied in the field after each sweep·set Insects, but. not. spiders, were classified as adult or immature. Spiders were identified only to order. Oriw;. Geocoris, and Nabis species were identified to genus; predatol)' coccinellids were recorded as "lady beetles"; chrysopids and hemerobiids were recorded as "lacewings"; and mantids, reduviids, and predatory pent.alomids were grouped as "others." Because the sweep-net method was used, no attempt was made to record l)fedator eggs or ground predators.

Statistical analysis was performed on the sample unit. (10 sweeps). Data were grouped by week, by weed, and by t.reatment (mowed/unmowed). Factorial analyses of variance on t.he raw density dat.a and on two separate t.ransformations t.hereof produced similar statistical relationships; so results from the untl'ansfonned data are reported. Means are weighted for unequal sample sizes. All statements of statistical significance herein are made at P = 0.05. Data from the two years were analyzed separately.

RES LTS AND DISCUSSIOI

Mean densities of the various predators in the mowed and unmowed areas, averaged across the sampling period and across weed species, are shown in 'fable 1. Although there were approximat.ely twice as many predators in 1982 than 1983, total predator density in bot.h years was significantly lower in the mowed than in the unmowed ' ...·eeds (by 17(% in 1982 and by 32% in 1983). However, mowing did not affect. all predator groups uniformly. No significant reductions in the mowed area were observed in either year for DrillS immatures, lacewing adults, Nabis adults and immatlll'es, 01' for "other" adults and immatures. But in both years in mowed weeds the densit.ies of adult and immature lady beetles. immature Geocoris, and immature lacewings were significantly lower than in unmowed weeds, by an average of al>proximately 52%. The remaining groups (spiders, Drius adults, and Geocoris adults) showed significant differences in one but not the other year. In 1982, Orius adult density was actually higher (by 26%) in the mowed weeds.

We had postulated that non· flying predatory arthropods, i.e. spiders and immature insects, may more likely be reduced in density by mowing than would adult insects. The latter could conceivably flyaway at the disturbance of oncoming mowers 01', if not, surviving adults could flyaway from mowed plants. The data do

80 J. Agric. Entomol. Vol. 3, l\o. 1 (J 986)

not totally support the assumption (Table 1). Non·nying predat.or density was 26% lower in mowed than unmowed weeds in 1982 and 28% lower in 1983. The density of adult insects was essentially the same in the mowed and unrnowed weeds in 1982. but in 1983 their density was 40% lower in the mowed weeds.

Table 1. Effect of mowing on density of various predatory arthropods in spring weeds in southeastern Arkansas.

X individuals/lO sweeps'"

1982 1983

Predator categoryt Mowed Unmowed Mowed Unmowed

Spiders 6.4 7.5 :1.6 4.0

Drius A's 4.3 3.2 1.2 1.8

Orius I's 2.6 2.8 1.0 1.2

Lady beet.le A's 1.5 2.4 0.6 1.4 Lady beetle I's 1.8 3.6 1.2 2.4 Geocoris A's 0.4 0.4 0.3 0.5

Geocoris rs 0.3 0.8 0.2 0.6 Lacewing A's 0.1 0.2 O.J O.J

Lacewing I's 0.2 0.5 0.3 0.6 Nabis A's 0.4 0.3 O.J 0.1

Nabis I's 0.1 0.1 < 0..1 < 0..1

Other A's < 0.1 0.1 < 0.1 < 0.1

Other I's 0.1 0.1 0..1 0.1

All predatory arthropods J8.2 22.0 8.8 13.0 Predatory insect A's 6.7 6.6 2.4 4.0

Predatory insect I's 5.1 7.9 2.8 4.9 lon·nying predators 11.5 15.4 6.4 9.0

• Means joined by a common underscore are nOl significalltly different under an F' lest (P- O.~). t A's = adults; rs - immatures.

A graphical representation of week-by·wcek density for tot.al predators in the two areas (Fig. I) shows that in 1982 the reductive effect of mowing was confined to a 3-wk period in late ApriVearly May. In 1983, the lower density in the mowed weeds prevailed more uniformly across the sampling period.

Seventeen weed types were sampled, but data are reported separately for only three - crimson clover (Trifolium incarnatum Linnaeus), gel'Dnium (Geranium carolinianum Linnaeus). and two species of vetch which were treated as one host in this study (Vicia saliva Linnaeus and V dasycarpa Tenore) - because these species supported the highest Heliot,his densities (Harris and Phillips, unpublished data). Vetch and crimson clover were also the most common dicot weeds in the area. The other 14 weed types are grouped under "other weeds." When all weed species were considered together (Table 1). total predator density was significantly lower in the mowed vs. the unmowed weeds in both years. When the four weed categories were considered separately (Table 2), only crimson clover in 1982 and

36 1982 ~ 19831\ mowed, \

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MARCH APRIL MAY JUNE APRIL MAY JUNE

Fig. 1. Mean weekly densities of predatory arthropods in mowed and unmowed spring weeds in southeastern Arkansas, 1982 and co1983. First Heliothf;j larvae were found in spring weeds in weeks of 19 April 1982 and 2 May 1983.

8

82 J. Agric. Entomol. Vol. 3. :-.la. I (1986)

Table 2. Effect of mowing on density of predatory arlhropods in various spring weeds in southeast.ern Arkansas.

X individuals/IO sweeps·

1982 1983

\Veed categol)' Mowed Unmowed Mowed Unmowed

Crimson clover 15.9 (141) 32.4 (l12)! 9.7 (159) 11.9 (165)

Geranium 21.2 (26) 17.8 (35) 7.6 (34) 9.2 (31)

Vetch 26.6 (91) 27.2 (96) 10.4 (91) 17.4 (109)

Other weeds 15.8 (217) 15.8 (243) 7.4 (176) 11.8 (175)

• Means joined by a common underscore are nOl si~nificolltl)' diffcrcnl under nn r test <P- 0.0;). t Values ill parentheses nrc lh(' number of SWCClrseu (10 sweeps/set) from which each mean is

derived.

vetch and other weeds in 1983 showed signiricant.Iy lower total predator densities in association with mowing.

Mowed/unmowed comparisons for each predator and weed category are shown in Fig. 2 (l982) and Fig. 3 (1983). These figures also facilitate a visual comparison of the relative abundance of the different predators, although no statistical comparisons are made among predators because the various categories were analyzed separately. Note the different scales for the two years. Orius adults in other weeds in 1982 and in crimson clover in 1983 were significantly higher in the mowed area. All other significant differences shown by asterisks in Fig. 2 and 3 reflect significantly lower predator densities in the mowed area.

Crimson clover was the weed host in which mowing significantly reduced the density of the greatest number of predator categories. Of the 13 categories, four showed significant density reductions in both years and six in one or the other year. However, in 1983 in crimson clover, the densities of three relat.ively abundant predator groups - spiders, Orius adults, and Oriu.<; immatures - were actually higher in the mowed area, which explains why t.otal predator densit.y was not significantly lower in mowed crimson clover in j 983 (Table 2).

Vetch supported the highest density of predators in the study area (averaged over mowed and unmowed areas): 27 and 14 predators/IO sweeps in 1982 and 1983, respectively. Crimson clover, which supported the next highest, had 23 and 11/10 sweeps. Only four categories of predat.ors were significantly reduced in density in vetch in the mowed area, and each in only one year. A possible explanation for this is that some vetch clusters sometimes missed being mowed, as described earlier, because of location high up on [l steep railroad bank, for example. Crimson clover commonly grew in natter areas, where only thin strips would be missed between passes of a mower.

When years, wecd calegories, and adulis and immatul'cs ~ere considered together, t.he relative abundances of the various predator groups in X individuals/IO sweeps were, in decreasing order. spiders (5.4), Orius (4.5), lady beel!es (3.8), Geocoris (0.9), lacewings (0.6), Nobis (0.3), and other predators (0.1). Geocoris density showed the greatest reduction in mowed vs. unmowed weeds (50.4% reduction), followed by lady beetles (47.I % reduction), and then lacewings (45.5% reduction). Other reductions were less than 14%. In view of the much greater abundance of lady beetles in the study area during the two springs, compared with Geacoris or

2 4 6 B 2 4 6 B 10

VETCH

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1982 • mowed o unmowed

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ORIUS A's

ORIUS I's

LADY A's

LADY I's

GEDCORIS A's

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NABID A's

NABID I's

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CRIMSON CLOVER GERANIUM

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X Individuals/IO sweeps

Fig. 2. Relative densities of predatory arthropods in mowed and unmowed spring weeds in southeastern Arkansas, 1982. '" = significant density difference in mowed vs. unmowed weeds under F test (P= 0.05).

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1983 • mowed

o unmowed

2 3 4 6 I 2 3 2 3 4

X Individuals/IO sweeps

2 3

Fig. 3. Relative den$iitics of predatory arthropods in mowed and unmowed spring weeds in southeastern Arkansas, 1983. * = significant density difference in mowed vs. unmowed weeds under F t.est (P = O.OG).

HAnHlS nnd PHILLIPS: Effect of ~1owing Weeds on Predators 85

lacewings. we consider Ihe reduction in lady beetle density to be the most serious effect mowing: had on predatory arthropod fauna in sl>ring weeds. Spiders, the most abundant predlltors found, were reduced by only L3.4%, and Orills, the next mosl abundant., were not reduced.

CQ)lCLUSIONS

Densit.y reductions of predatory arthropods in mowed vs. unmowed weeds observed in this st.udy were statistically significant. However. the magnitudes of the reductions (177, in 1982 and 32% in 1983) were considerably lower than those observed for /-Ieliolhis larvae studied concurrent Iy. !\·fean densities of /-Ieliolhi$ larvae per 10 sweeps in mowed \'s. unmowed weeds were 0.4 vs. 0.2 (I982) and 1.0 vs. 0.5 (I 983). a 50'ft reduction each year (Harris and Phillips, unpublished data).

The lesser reductive effect on predatory arthropods than on Heliothis larvae is a point. in favor of mowing as a suppression tactic for control of /-Ieliothi... in spring weeds. Another control method often mentioned in theoret.ical exposilions of the rationale behind wide-urea, early-season suppression of these pests is the application of herbicides to spring weeds (Knipling L979; Stadelbacher 1982; I<nipling and Stadelbacher 1983; Mueller et al. ]984). This has not yet been tested in regard to impact. on predatory arthropods. However. it is generally believed t.hut widespread use of herbicides in spring weeds would be devastating to beneficial arthropods, because their habit.at. would be virt.ually destroyed for the entire spring after herbicide application. In t.his study mowing did not destroy the spring host plants; it merely retarded their growth.

The lower detrimcntal erfect or mowing on predators compared with Heiiothi.., in the spring generation may prove inconsequential ir, as cautioned by Mueller et al. (1984), the negat.ive impact of spring mowing on subsequent population growth of predatory arthropods were greater than on subsequent population growt.h of /-Ielio/his. This could cventuate, all else remaining equal, because of the lower potential rate or increase of most. predat.ors (lower fecundity und lower egg-laying rate) COml}ared wilh Heliothis. Field trials over several generations to test this hypothesis would be necessary.

Knipling and Stadclbachcr (1983) considered cultural control (mowing. cultivation. and use of selective herbicides) to be the most economical of the several potenlial met.hods they studied in their theoretical analysis of area·wide mungement of Helio/his in its first (sp,-ing) generation. They estimated cultuml control. with mowing as one component, would cost S25/ha. We agree with I<nipling and Stadelbacher (1983) that eml}loyment of just one tactic for wide·area. early-season suppression of Heliothis is ill-advised. Success should be more likely with a judicious combination of several t.actics. designed to greatly reduce Heliollu:o; larval populations in spling weeds, yet not inadvertently allow their subsequent resurgence in crops because of an even greater reduction of beneficial arthropod populations in the spring weeds.

86 J. Agric. Enloma!. Vol. 3, No. I (1986)

REFERENCES CITED

Knipling, E. F. 1979. The basic principles of insect population suppression and management. U. S. Dcp. Agric. Handbook 512. 623 pp.

Knipling, E. r., and E. A. Stadelbacher. 1983. The rationale for fll'CCl-wirle management of Heliothis populat.ions. Bull. Entomol. Soc. ArneI'. 29(4): 2D-37.

Mueller, T. F., V. E. Harris, and J. R. Phillips. 198·1. Theory of Hl!fio/.his (Lepidoptera: Noctuidncl mllllugement through reduction of the first :-;pring generation: a critique. Environ. Entomol. 13: 625-634.

Stadelbacher, E. A. 1982. An Dvelview and simulation of tacrics [or manngemcnt. of Heliothis spp. on early-season wild host plants. Proc. Belt.wide Cotton Pmd. Res. Conrs. Jan 3-7, 1982. Nat.iollfll Cot.ton Council of America. P.O. Box 12285, Memphis. TI\ 38112. pp. 209-212.