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HānaiʻAi 49 Jan|Feb|Mar 2023
Sustainable Pest and Soil Health Management for Sweet Potato Production
Melanie Pitiki1, Benjamin Wiseman1, Landon Wong1, Brent Sipes1, Joshua Silva2, Jensen Uyeda2 , Roshan Mandhar1, and
Koon-Hui Wang1
Department of Plant and Environmental Protection Sciences1, Department of Tropical Plant and Soil Sciences2, University
of Hawaii at Mānoa
Sweet potato (Ipomoea batatas) is an important vegetable crop in Hawaii with a farm
gate value of $2.24 mil in 2022 (NASS, 2023). However, widespread damage from multiple
pests and pathogens limits sweet potato production, especially when sweet
potatoes are grown for multiple years in the same location. Challenges related to
pest management are especially problematic for organic producers due to the
high costs and low efficacy of available organic pest control measures for sweet
potatoes. This project focused on exploring sustainable pest and soil health
management strategies that can help organic sweet potato producers improve
productivity.
a
Weevils and stem borers
b
c
Fig. 1 a) Sweet potato weevil, b) rough
sweet potato weevil and c) West Indian
sweet potato weevil (Credit a and c: R.
Myers, b: HDOA).
Among the arthropod pests that affect sweet potato, the sweet potato
weevil (Cylas formicarius, SPW), the West Indian sweet potato weevil
(Euscepes postfasciatus, WSPW) and the rough sweet potato weevil (Blosyrus
asellus, RSPW) are the most widespread and damaging in Hawaii (Fig. 1). Both
SPW and WSPW tunnel through the vine to reach storage roots buried in the soil and spend the majority of their life cycle
in the root, making spray contact on these pests difficult. Commercial SPW pheromone traps such as Pherocon unitraps
(Alpha Scents, Canby, OR) are available and can be added into an integrated pest management (IPM) program for SPW
management (McQuate and Sylva, 2014). The phermone traps can be used to monitor SPW populations and should be
placed 60 m apart, with 3 traps at equal distance per 40-acre field. The economic threshold for insecticide treatment is 4
weevils/pheromone trap per week (Jansson et al., 1991). Unfortunately, no pheromone trap is available for WSPW and
RSPW.
Sweet potato vine or stem borer, Omphisa anastomosalis (Fig. 2), is also a common pest of sweet potato in Hawaii.
Larvae bore into the stem and storage roots, creating cavities and causing wilting and death. Early infestation during the
vegetative phase can cause 30-50% yield losses. A pile of frass can be found under the attacked stem (Amalin and Vasquez,
publication year unknown).
Recommendations against weevils and stem borers include crop rotation for at least one year, planting new fields a
mile from old fields, post-harvest destruction of culls, intensive pesticide sprays and installing pheromone traps, hillingup of soil around the base of plants, sufficient irrigation to prevent soil cracking, as well as prompt harvesting to avoid a
dry period (Vasquez and Amante, year unknown). Manandhar et al. (2022) reported several sweet potato varieties in
Hawaii are more tolerant to these pests. Crop rotations should be made with non-host crops that do not foster weevil and
stem borers. In Hawaii, suitable rotational crops include corn, turmeric, and pasture if integrating livestock with sweet
potato production. Corn and many grasses used in pasture are non-hosts to reniform nematodes (Wang, 2007). To avoid
pest hosts during the rotation, sweet potato volunteers or related weeds need to be avoided during the rotation period.
The amount of time and land needed to effectivly use crop rotation for pest control makes it difficult for small-scale
farmers to utilize crop rotation. Insecticides can help limit pest disease, but organic farmers have limited insecticides
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Volume 51 July |August| September 2023
available to combat this weevil complex. Available organic pesticides include the bioinsecticides Beauvaria bassiana and
Metarhizium anisopliae. However, research is needed to improve the efficacy of bioinsecticides against these pests.
Fig. 2. a) A sweet potato stem borer larva bores into the stem of sweet potato, b) the point of penetration of stem
borers into the vines close to or into the storage roots leaving a pile of frass , c) damage by stem borer to the tip of the
sweet potato, and d) hollow cavities in the storage root caused by stem borers (credit: L. Wong, B. Wiseman).
Plant-parasitic Nematodes
Fig. 3. Damage from nematodes on sweet potato includes a) cracking on the storage roots, b) galls formation on the lateral
roots cause by root-knot nematodes, and c) slender storage roots despite healthy vegetative growth. d) A root-knot
nematode female producing egg masses outside of a gall. e) a reniform nematode female is kidney shaped but does not
form galls on the roots (credit: C. Shloemer and K.-H. Wang).
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Plant-parasitic nematodes are another major pest of sweet potato. In Hawaii, root-knot nematode (Meloidogyne spp.)
and reniform nematode (Rotylenchulus reniformis) are two common genera infecting sweet potato. Effective control of
root-knot nematodes by a synthetic nematicide, fluopyram, increased the marketable yield of sweet potato by 6.3-fold
compared to the untreated control (Waisen et al., 2021). Effective management of reniform nematode populations by
pre-planting of sunn hemp cover crop followed by monthly application of Molt-X (a.i. azadirachtin), a neem product, did
not significantly improve sweet potato yield (Waisen et al., 2021). More research is needed to identify effective organic
management approaches for plant-parasitic nematodes on sweet potato.
Tropical Cover Crops for Sustainable Sweet Potato Production
We implemented cover cropping and other conservation agricultural practices in an organic sweet potato production
trial and examined soil health, pest control, and other beneficial environmental outcomes. Managing soil health is more
difficult for sweet potatoes compared to other row crops due to cropping practices that require hilling of planting beds
and harvesting procedures that require deep digging of the storage roots. This means constant soil disturbance is
inevitable. Since we cannot harvest sweet potato crops using no-till techniques, rotating cover crops with sweet potato
and terminating the cover crops with strip-till practice might be a critical step to restore soil health for sweet potato. Four
tropical cover crops with nematode allelopathic (suppressive) effect were tested (Fig. 4).
Fig. 4. Four tropical cover crops possessing nematode allelopathic compounds: a) sorghum contains dhurrin, b) sunn hemp
contains monocrotaline, c) marigold contains -terthienyl, and d) velvet bean contains L-DOPA (dopamine) (picture credit:
K.-H. Wang).
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Besides an ability to suppress plant-parasitic nematodes, proper cover crop selection could also overcome soil health
degradation over time from constant tillage. Soil health is “the continued capacity of the soil to function as a vital living
ecosystem that sustains plants, animals, and humans” (NRCS). Healthy soil is supported by a diverse arena of soil
microorganisms that form a functional soil food web that can sustain soil nutrient cycling and other ecosystem functions.
For example, soil bacteria and fungi contribute to plant growth regulation, nutrient cycling, and carbon sequestration.
Symbiotic fungi, like arbuscular mycorrhizal fungi (AMF), assist plants to absorb soil phosphorus.
Soil microbial communities can be examined through various biological tests. Since all soil microorganisms contain
phospholipid fatty acids, measurement of phospholipid fatty acids (PLFA) of the total microbial community in the soil can
estimate the “living” biomass, broad microbial community structure, and environmental stress impacts on soil health. In
addition, many researchers have shown that free-living nematodes are reliable soil health indicators (Ferris et al., 2001).
Nematodes are ubiquitous, functionally diverse, and well classified into functional groups (Yeates et al., 1993). Further,
nematodes are easy to sample and play an important role in soil nutrient cycling. Free-living nematodes directly influence
soil processes and reflect the structure and function of many other taxa within the soil food web. Nematode communities
are sensitive to changes in soil quality and the frequency of soil disturbances. Thus, nematode community analysis can
provide an indication of how pest and soil management practices are affecting soil health (Ferris et al., 2001).
Field Study
A field trial was conducted at Poamoho Experiment Station, University of Hawaiʻi in Waialua, HI where cover crops
‘NX-D-61’ energy sorghum (Sorghum bicolor, Koolau Seed Supplies, HI), ‘Tropic Sun’ sunn hemp (Crotalaria juncea, Oahu
RC&D, Kunia, HI), velvet bean (Macuna pruriens), and ‘Nema-Gone’ marigold (Tagetes patula, Burpee, Warminster, PA)
were grown for 3 months (from September 1 to December 1, 2022) and compared to a bare ground control prior to sweet
potato planting. To determine how long each cover crop should be irrigated to maximize cover crop biomass production,
each cover crop received 3 irrigation regimes: 2, 4 or 8 weeks of 4 hours drip irrigation (approximately 120 K gal
water/acre/week). Each field plot was 10 × 5 ft2 in size and each treatment was replicated in 4 plots arranged in a
randomized complete block design. Biomass of cover crops were estimated at termination. All cover crops were
terminated by flail mowing followed by strip-tilling a 2-ft wide strip in the middle of each plot to 4 inches deep with a
rototiller. ‘Okinawan’ sweet potato cuttings were planted on December 8, 2022 only in the 8-week irrigated plot to
compare cover crop effects on sweet potato
cultivation. Soil samples were collected prior to
cover crop planting and 2 weeks, 3 and 5 months
(at harvest) after sweet potato planting. A SPW
Pherocon unitrap was installed in the middle of
the field 1 month after planting to reduce SPW
pressure. Each cover crop plot was split into half.
Half was treated with a foliar spray of Beauvaria
bassiana (Mycotrol, Certis USA L.L.C., Columbia,
MD) at monthly intervals beginning at 2.5
months after planting whereas the other half
was not treated.
Results and Discussion
Irrigation Needs of Cover Crops: Among the
cover crops tested, sorghum and velvet bean
produced the most biomass (P ≤ 0.05), especially
if irrigated weekly for 8 weeks. However, velvet
bean was the only cover crop that produced
equivalent biomass between 4 and 8 weeks of
weekly irrigation. This means that velvet bean
Fig. 5. Cover crop biomass generated in bare ground (BG), marigold (MG),
sunn hemp (SH), velvet bean (VB) and sorghum (SG) plots after 4 months of
planting. Each cover crop was split into 3 subplots that received weekly
irrigation for 2, 4 or 8 weeks. Means of cover crops (average among irrigation
treatments) marked with the same letter were not different based on WallerDuncan k-ratio (k=100) t-test.
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irrigation can be terminated at 4 weeks after planting
without sacrificing cover crop biomass production
compared to 8 weeks of irrigation (Fig. 5). Under the 8weeks irrigation regime, sorghum generated 55 tons/acre
whereas velvet bean generated 38.8 tons/acre of biomass,
and both added significant amounts of biomass to the soil.
Effects on Soil Properties: The velvet bean plots had
significantly increased soil carbon (C) content 2 weeks after
strip-tilling of the velvet bean compared to the bare
ground (Fig. 6A). Though not significantly different from
the other cover crops, planting of VB resulted in the
highest water infiltration rate throughout the sweet
potato cropping cycle (Fig. 6B). All soil samples collected
throughout the sweet potato cropping cycle were also
subjected to Solvita Labile Amino-Nitrogen (SLAN) test
which reports organic nitrogen reserves present as aminosugars in soil. VB was the only treatment that increased
ammonia-N in the soil compared to BG (P ≤ 0.05, Fig 6C)
indicating a higher pool of organic N that is potentially
plant available.
Effects on Soil Microbial Biomass: Soil samples were
collected from the rhizosphere of sweet potato from 3
plants/plot at 2 weeks and 3 months after sweet potato
planting and submitted for phospholipid fatty acid (PLFA)
analysis at Regen Ag Lab (Pleasanton, NE). This analysis
showed that VB increased microbial diversity, gramnegative bacteria, total fungi, arbuscular mycorrhizal fungi
(AMF) biomass, and fungi: bacteria ratio (F/B) (P ≤ 0.05,
Table 1), but lowered actinomycete (ACT) microbial
biomass compared to some other treatments. These
results indicate a more diverse soil microbial community
and less stressful (nutrient depleted) soil in the VB plots,
and these changes may be due in part to higher SLAN in VB
treated soil.
Fig. 6. A) Total soil C content (n=8), B) soil infiltration rate
(n=8) and C) soil ammonia-nitrogen measured by Solvita
SLAN test (n=20).
Table 1. Effect of cover crop on soil microbial biomass on sweet potato based on phospholipid fatty acid (PLFA) analysis.
ACT= Actinomycetes, GN = gram negative bacteria, AMF = Arbuscular Mycorrhizae Fungi, F/B = Fungal/Bacterial PLFA
biomass. Means (n=8) in a column with the same letter are not different based on Waller-Duncan k-ratio t-test (P ≤ 0.05).
Trt
Total (ng/g)
Diversity
BG
MG
SG
SH
VB
1585.78
1584.07
1956.59
1979.20
2227.05
1.11 b
1.14 b
1.16 b
1.13 b
1.30 a
ACT (ng/g)
140.51a
132.43a
153.45ab
157.60ab
141.66b
GN (ng/g)
Fungi (ng/g)
AMF (ng/g)
F/B
120.28 b
145.73 b
217.39 ab
201.29 ab
288.49 a
13.05 b
51.80 ab
27.96 b
22.70 b
105.96 a
0.00 b
3.79 b
4.86 b
0.40 b
33.06 a
0.03 b
0.06 b
0.04 ab
0.03 b
0.13 a
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Effects on Plant-parasitic Nematodes and Free-living Nematodes:
Reniform nematodes (Rotylenchulus reniformis) were the dominant
plant-parasitic nematode at the field site. At sweet potato planting, the
average populations of plant-parasitic nematodes were less than
100/250 cm3 soil in all treatments. By the time the sweet potatoes were
harvested, the average population densities of plant-parasitic
nematodes ranged from 1000 to 3000/250 cm3 soil, depending on the
preceding cover crop. The data suggested that sunn hemp and velvet
bean contributed to decreasing the final nematode populations (P ≤ 0.10,
Fig. 7). Effects of the cover crop on the plant-parasitic nematode
population will be examined again in the second cropping cycle when the
starting populations of plant-parasitic nematodes are expected to be
higher. Examination of the free-living nematodes and calculated
nematode-derived soil health indicators did
not show differences among the cover crop Fig. 7. Population densities of plant-parasitic nematodes (n=4) at sweet
treatments.
potato harvest, separated by cover crop that preceded sweet potato.
Effects on weevils and yield: Sweet
potatoes were harvested 5 months after
planting. Based on the grading standards for
Hawaii-grown sweet potatoes (Hawaii
Department of Agriculture, Marketing and
Consumer Services Division, Commodities
Branch, 1986), the total marketable yield of
sweet potatoes was not different among the
cover crop treatments (P > 0.05). Depending
on the cover crop treatment, between 26-51%
of the roots were damaged by RSPW, 9 -13%
by SPW, and only 0-7% by plant-parasitic
nematodes (Fig. 8). VB was the only treatment
that reduced the damage of sweet potato
roots from RSPW compared to the bare ground
control (P ≤ 0.05, Fig. 9A), and no differences in
SPW and nematode damage was detected
among the cover crop treatments.
Fig. 8. A) Numerous pitted holes caused by sweet potato weevils, B)
skin scarification caused by rough sweet potato weevils, and cracking
and small storage roots indicative of reniform nematode infection
(credit a, b: M. Pitiki, c: K.-H. Wang).
Fig. 9. A) Effects of pre-plant cover
crops on sweet potato roots
damaged by rough sweet potato
weevil (RSPW), and B) effects of
Beauvaria bassiana application on
sweet potato roots damaged by
sweet potato weevils (SPW) at
harvest. Means (n=8 and n=20 for
A and B, respectively) followed by
a different letter or marked with
** indicate significant level at P ≤
0.05 and P ≤ 0.01, respectively).
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The monthly foliar application of Mycotrol (B. bassiana)
reduced SPW damage by 80% (P ≤ 0.01, Fig. 9B) compared to
untreated plots when both plots were in the presence of a SPW
pheromone trap. While this is an encouraging result for
managing SPW, the Mycotrol treatment did not affect RSPW
damage.
At one and two months after planting, we monitored the
occurrence of B. bassiana and M. anisopliae, two natural fungal
enemies of the three weevils and stem borer. Field cages filled
with field soil and baited with 5 wax worm (Galleria mellonella)
larvae, buried 2 inches (5 cm) deep in the VB and BG plots for 1
week, and brought into the lab to observe and record incidence
of wax worm larvae colonization by B. bassiana (Bb) or M.
anisopliae (Met) 2 weeks after lab incubation. Approximately 30% Fig. 9. Incidence of wax worm baits colonized by
of the wax worms were either colonized by Bb or Met over the Beauvaria bassiana (Bb) or Metarhizium anisopliae
two sampling times, but no colonization of these (Met) in field cages buried in velvet bean (VB) or bare
entomopathogenic fungi was observed on the BG plots, ground (BG) plots. Means (n=8) with letters indicate
suggesting that VB treatment might have enhanced the
significant differences based on Waller-Duncan k-ratio
colonization of Bb and Met in the soil. Future research will
(k=100) t-test.
investigate consistent performance of VB in enhancing EPF in the soil.
Summary
This field trial demonstrated that planting velvet bean prior to sweet potato planting is a promising tropical cover crop for
sweet potatoes. Velvet bean:
•
•
•
•
•
was the most water efficient in generating biomass compared to the other cover crops and generated a similar
amount of biomass with 1 or 2 months of irrigation.
increased total soil C and soil labile amino-nitrogen (SLAN) in one cropping cycle.
fostered a more diverse and less stressful soil community as evidenced by increased soil microbial diversity, gramnegative bacteria, total fungi, arbuscular mycorrhizal fungi biomass, and fungi: bacteria ratio, while reducing
actinomycete (ACT) microbial biomass.
reduced the proliferation of plant-parasitic nematodes in the soil during the sweet potato growing season.
reduced the damage of sweet potato roots from rough sweet potato weevils and increased the colonization of
soil insects by indigenous entomopathogenic fungi such as B. bassiana and M. anisopliae.
This research also demonstrated that an integrated pest management strategy combining the SPW pheromone trap and
monthly foliar spray of B. bassiana during the sweet potato root formation stage provided a promising organic approach
to manage SPW. Future work is needed to improve management of other weevils and stem borer pests of sweet potatoes.
Literatures Cited
Ferris, H., Bongers, T., and deGoede, R.G.M. 2001. A framework for soil food web diagnostics: extension of the nematode
faunal analysis concept. Applied Soil Ecology 18: 13-29.
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Volume 51 July |August| September 2023
Hawaii Department of Agriculture, Marketing and Consumer Services Division, Commodities Branch. 1986. Standards for
Hawaii-Grown Sweet Potatoes. Hawaii Administrative Rules, Standards for Fresh Fruits and Vegetables.
https://hdoa.hawaii.gov/qad/files/2012/12/AR-41-57.pdf
Jansson, R. K., L. J. Maon, and R. R. Heath. 1991. Use of sex pheromone for monitoring and managing Cylas formicarius,
pp. 97-138. In R. K. Jansson and K. V. Raman (eds.) Sweet Potato Pest Management: A Global Perspective. Westview
Press, Inc., Boulder, Colorado. p. 458. https://doi.org/10.1201/9780429308109-6.
McQuate, G.T. and C.D. Sylva. 2014. Trapping sweetpotato weevil, Cylas formicarius (Coleoptera: Brentidae), with high
doses of sex pheromone: Catch enhancement and weathering rate in Hawaii. Proceedings of the Hawaiian
Entomological Society 46: 59–69. https://hdl.handle.net/10125/25.
Manandhar, R., J. Keach, and E. Kirk. 2022a. Three promising sweet potato varieties for Kauaʻi from a 2019 trial. Hāina’Ai
45 : 7 pp. https://gms.ctahr.hawaii.edu/gs/handler/getmedia.ashx?moid=71115&dt=3&g=12.
NRCS. Publish year unknown. Soil health. https://www.nrcs.usda.gov/conservation-basics/natural-resourceconcerns/soils/soil-health#assistance (last access on September 5, 2023).
NASS 2023. Hawaii 2022 vegetable crop value at $73.0 million. Pacific Region-Hawaii vegetable and melon crops report.
https://hdoa.hawaii.gov/add/files/2023/02/202302vegrv.pdf.
Reddy, G.V.P., S. Wu, R.C. Mendi, and R. H. Miller. 2014. Efficacy of pheromone trapping of the sweet potato weevil
(Coleoptera: Brentidae): Based on dose, septum age, attractive radius, and mass trapping. Environmental Entomology
43: 767-773.
Vasquez, E. and V. Amante. Year unknown. Sweetpotato stemborer. in J. O’Sullivan, V. Amante, G. Norton, E. van de Fliert,
J. Pardales, E. Vasquez (eds.) Sweetpotato DiagNotes: A Diagnostic Key and Information Tool for Sweetpotato
Problems.
https://keys.lucidcentral.org/keys/sweetpotato/key/Sweetpotato%20Diagnotes/Media/Html/TheProblems/PestRoot&StemInsects/SPStemborer/Sp%20stemborer.htm (Accessed Aug 16, 2023).
Waisen, P., K.-H. Wang, J. Uyeda, and R.Y. Myers. 2021. Effects of fluopyram and azadirachtin on plant-parasitic and freeliving nematodes on zucchini, tomato and sweet potato. Journal of Nematology 53: 1-15.
https://doi.org/10.21307/jofnem-2021-030.
Wang, K.-H. 2007. Reniform nematodes. Featured creature EENY-210. IFAS, University of Florida.
https://entnemdept.ufl.edu/creatures/nematode/r_reniformis.htM.
Yeates, G.W., Bongers, T., de Goede, R.G.M., Freckman, D.W. and Georgieva, S.S. 1993. Feeding habits in soil nematode
families and genera-an outline for soil ecologists. Journal of Nematology 25: 315–331.
Acknowledgement
This project is supported by NIFA OREI (Award 20215130035225), CTAHR Hatch, Multistate (NE2140) and Plan of Work
(HAW9048-H, 9034-R, POW 16-964, and POW 22-037). We also like to thank Roshan Paudel, Kekoa Larger and Quynn Cytryn for
technical assistance. Mahalo to Oahu County staff for field support.
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