Comparison of Alternative Management
in Fall Lettuce
John C. Palumbo
For the second year, a large block experiment was conducted at the Yuma Ag Center to
compare the field performance of three lettuce management programs for control of
lepidopterous larvae. Conventional, experimental and biorational insecticides were sprayed
to control beet armyworm, cabbage looper and Heliothis species
throughout the growing
season. Differences in populations of total larvae among the four treatments, relative to
insecticide treatments and timing of application were observed throughout the season. In
general, the standard and experimental treatments provided the most consistent control of
lepidopterous larvae following each application. Harvest data showed that the spray
regimes had a significant influence of head lettuce yield or quality. Maturity and quality
were significantly reduced in the untreated control. An economic analysis shows that net
returns varied widely among the management programs at different market prices. In
conclusion, this study provides preliminary data to support the need for more development
of experimental and biorational insecticide products as alternatives to conventional
management programs in desert lettuce production.
The beet armyworm, cabbage looper and Heliothis species are
the major lepidopterous
pests of lettuce in desert growing areas of Arizona (Kerns and Palumbo 1997) . Standard
insecticides such as methomyl (Lannate) and thiodicarb (Larvin) combined with pyrethroids
have been successfully used in controlling this pest complex over the past several years
(Palumbo et al. 1993, Palumbo et al. 1994, Palumbo 1996). These products are used
frequently during the season, and many speculate that there field effectiveness will soon
be reduced under these use patterns. Unfortunately, there are no alternatives insecticides
which offer comparable control of lepidopterous larvae. Furthermore, with the recent
passage of the Food Quality Protection Act of 1996 (Schreiber 1996), there is speculation
that some of the more broadly toxic compounds may be removed from the market in the next
few years.The organophosphate and carbamate insecticides (ie. Lannate, Orthene, Larvin,
Endosulfan, Monitor, Diazinon) are being targeted as prime candidates for reduced usage.
There are several alternative products available to lettuce growers
for management of the
lepidopterous complex. However, these consist of biological (Bts), botanical (neem
pyrethrum, rotenone) and inorganic (cryolite) insecticides that have been demonstrated to
have only marginal activity on beet armyworm and cabbage looper in lettuce (Palumbo et al.
1992, Palumbo 1995). There activity against Heliothis is not
known. Several new compounds
are currently being developed which offer not only excellent efficacy against these pests,
but are also new insecticide chemistries (Palumbo and Kerns 1996). However, we are
uncertain how these alternatives will perform in the absence of the conventional
insecticide products under desert growing conditions. Therefore a large- block field study
was conducted to compare worm management in head lettuce using conventional, experimental
and biorational compounds.
Materials and Methods
Lettuce, Lactuca sativa L., 'Early Queen' was direct seeded into large blocks on
double-row beds on 9 Sep at the Yuma Valley Agricultural Center, Yuma, Az. Each plot
consisted of twelve beds, 200 ft long beds spaced 42 inches apart and bordered on each
side by two untreated beds. Plots were thinned to a stand on 30Sep. Plots were arranged in
a completely randomized block design with 3 replicates. Treatments consisted of
experimental, biorational and conventional lettuce management programs compared to an
untreated control (Table 1).
Applications were made on an as needed basis (Table 1). All chemicals were applied by a
tractor-mounted boom sprayer operated at 60 psi and 30 gpa. Three, disc-type cone nozzles
were used per bed. All chemicals included spreader sticker (Latron CS-7) at a rate of
0.125% of the total volume. Evaluation of lepidopterous larvae control was based the
number of live larvae per plant sampled from the center 8 rows of each replicate at 1-3
times per week. The number of samples per replicate varied throughout the season,
decreasing as plant size increased (Table 2). The sample unit consisted of examination of
whole plants for presence of beet armyworm (BAW), Spodoptera exigua, Cabbage looper
(CL) , Trichoplusia ni , and Tobacco budworm (HEL), Heliothis virescens
larvae. Each species was characterized as large or small larvae. For BAW and HEL, larvae
were considered small if <5 mm in length, large >5mm. For CL, larvae were considered
small if <10 mm, large if > 10 mm. The number of Cabbage looper and Heliothis eggs
and beet armyworm egg masses per plants were also recorded during the first 50 days of the
The effects of larval feeding on plant stand, plant size and growth for each treatment
were estimated at pre-thinning (29 Sep), post-thinning (14 Oct), and at pre-harvest (18
Nov) stages. Plant stand was taken by measuring the number of plants within 100 ft on 2
beds within each plot. Plant size and growth was estimated by measuring dry weights (g).
Yields were taken when >75% of all heads in the experimental block were considered
ready for harvest on 19November (71 days after planting). Yields was taken by weighing all
heads within 30 ft of bed in 3 locations within each replicate. Quality was measured by
estimating the % marketable heads based on shape and firmness for each head harvested. If
heads maintained a reasonable shape and firmness they were considered marketable (#24s).
Each head was also evaluated for the presence of worm feeding damage and contamination.A
partial budget analysis was used to conduct an economic assessment of each lettuce
Results and Conclusions
Differences in populations of total larvae among the four treatments were observed
throughout the season (Figure 1). In general, the conventional and experimental programs
provided the most consistent control of lepidopterous larvae following each application.
The seasonal population fluctuations for each Lepidopterous species relative to management
program is shown in Fig 2-4. Cabbage looper was most abundant during the study peaking
after thinning and again near harvest (Figure 2). All three management programs provided
similar control of large larvae, but the conventional program appeared to have the
greatest residual control of small larvae. At 7-d pre-harvest, both the experimental and
biorational programs had significantly higher larvae numbers than the conventional. As
expected, beet armyworm numbers were high during stand establishment (pre-thinning). The
experimental management program provided excellent control during this period (Figure 3).
Unexpectedly, beet armyworm numbers peaked during the pre-harvest period. Similarly, the
biorational program did not prevent larvae from infesting heads at harvest when compared
with the experimental and conventional management programs (Table 3). Heliothis number
were surprising high in 1997, especially prior to heading (Figure 4). Again the
biorational program was inconsistent at holding larval numbers below damaging levels after
plants began to head.
Lepidopterous larvae had a significant impact on plant stand and plant growth (Table
4). Prior to thinning the plant stand, plant densities in the conventional plots were most
consistent. Plant densities in the biorational plots were significantly lower and larval
numbers were higher than the conventional program. As a result of high larval densities,
plant stand and plant size were significantly reduced in the untreated plots. After the
stand was thinned, all programs had higher plant densities and dry weights when compared
to the untreated plots. Larval numbers in the biorational plots were not different from
the untreated plots, but this did not effect plant density or plant size. At harvest,
lettuce plants in the biorational plots were significantly lighter than in the
Harvest data showed that the management programs had a significant influence of head
lettuce yield and quality (Table 4). Maturity and
quality were significantly reduced in the untreated control. The conventional management
program yielded significantly greatest number of cartons than the other management
programs when based on average heads/plot, % marketable heads, % worm damage and % head
contamination (Table 5). The biorational management
program was by far the most expensive, relative to net carton production. Consequently ,
the economic analysis shows that net returns varied widely among the management programs
at different market prices (Table 6.). At a low market price ($5.00/cart) none of the
treatments were profitable. At a more common market price ($7.50/cart) the biorational
treatment was not profitable, whereas at a market of $10.00 all spray treatments were
profitable. In conclusion, this study provides preliminary data to support the need for
more development of experimental and biorational insecticide products as alternatives to
conventional lettuce management programs under desert growing conditions.
- Kerns, D.L. and J.C. Palumbo. 1997. Lettuce IPM: Southwestern USA. In E.B.
Radcliffe and W.D. Hutchinson [eds.], Radcliffes IPM World Textbook,
URL:http://www.ent.agri.umn.edu/academics/classses/ipm/impsite.htm. University of
Minnesota, St. Paul, MN.
- Palumbo, J.C. 1995. Efficacy and temporal mortality of selective insecticides on beet
armyworm larvae in lettuce, pp 130-136. In N.F. Oebker (ed) 1994-1995 Vegetable
Report. University of Arizona, College of Agriculture Series P-100.
- Palumbo, J.C. 1996. Efficacy of Pyrethroids and lannate on lepidopterous larvae in
lettuce. Arthropod Management Tests, 21: 124.
- >Palumbo, J.C. and D.L. Kerns. 1996. Temporal activity of new insecticides
chemistries against beet armyworm in lettuce,143-146. In N.F. Oebker (ed) 1994-1995
Vegetable Report.University of Arizona, College of Agriculture Series P-104.
- Palumbo, J.C., C.H. Mullis, and F. Reyes. 1992. Evaluation of biological control of beet
armyworm and cabbage looper in lettuce, 1991. Insecticide and Acaricide Tests 17:115-116.
- Palumbo, J.C., C.H. Mullis, and F. Reyes. 1993. Evaluation of insecticides for control
of lepidopterous larvae inlettuce. Insecticide and Acaricide Tests, 18:133-134.
- Palumbo, J.C., C.H. Mullis, and F. Reyes. 1994. Evaluation of insecticides for control
of lepidopterous larvae in lettuce. Arthropod Management Tests 19: 101-102.
- Schreiber, A. 1996. Food Quality Protection Act, Part I. Agrichemical and Environmental
News, Issue No.130, Dec 1996, p.2-4.
Table 1. Insecticides, rates and dates applied for each lettuce management program,
YAC, Fall 1997.
Table 2. Dates, plant stages, and sample numbers on each collection date, YAC, Fall
Table 3. Numbers of larvae found on lettuce wrapper leaves and heads at harvest, YAC,
Table 6. Yields per acre and costs associated with various spray regimes for management
lepidopterous pests on lettuce, YAC, Fall 1996.
atotal number of cartons (24s) that could have been potentially packed from
field based on ave no. heads per 20 ft (see Table 3).
b average number of cartons (24s) that were harvested from plots after
individual heads were culled out due to shape, maturity, worm damage and contamination in
heads (see Table 3).
c see Table 1.
d based on average retail value from 3 local distributors of foliar chemicals
applied in each program (see Table 1 for products applied.).
e based on cost to apply insecticides by ground ($12.00 for banded application;
$11.00 for broadcast application). The first application (10 Oct) was applied as a band,
all subsequent sprays were broadcast.
fTotal cost of all chemicals applied during season; includes Insecticide and
application cost above cost, and $62.50 for Admire at planting applications (16 oz/acre)
and application of Karate (3.8 oz) and Diazinon (1pt) during stand establishment applied
to all treatments.
Table 7. Economic assessment of three lettuce management programs, YAC, Fall 1996.
This is a part of publication AZ1101:
"1998 Vegetable Report," College of Agriculture, The University of Arizona,
Tucson, Arizona, 85721.
This document located at http://ag.arizona.edu/pubs/crops/az1101/az1101_3.html
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