Observations on how cowpea aphid (Aphis craccivora) affects alfalfa of differing fall dormancy categories and some possibly resistant varieties

              STATE
UNIVERSITY
Observations On How Cowpea Aphid {Aphis craccivora) Affects Alfalfa of
Differing Fall Dormancy Categories and Some Possibly Resistant Varieties
Research Report 772
Leonard M. Lauriault, Dawn M. VanLeeuwen, and Rex E. Kirksey'
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ABSTRACT
In a study in the Southern High Plains of the USA designed
lor other purposes, a cowpea aphid (Aphis craccivora) infestation in April 2001 provided information that might be of
value to alfalfa breeders and managers and Cooperative Extension Service personnel, as this species can inflict severe damage to alfalfa (Medicago sativa L.), causing significant stand
loss. Cowpea aphid damage ratings ot furrow-irrigated alfalfa
oi tall dormancy (FD) categories 2 through 9, each represented by one to three varieties at Tucumcari, NM, were compared in a split plot with eight randomized complete blocks.
Fall dormancy category was a significant (P<0.05) factor in
the level of cowpea aphid damage, with FD 2 to 7 not significantly different and an increase in damage from FD 7 to 9.
There also were significant differences (P<0.05) for cowpea
aphid damage between varieties within FD 7(14 and 40% for
'Dona Ana' and 'Helena 7000', respectively) and within FD 9
(64 and 35% for 'Salado' and 'WL612', respectively). Using
more dormant alfalfa varieties in the Southern High Plains
and Southern Great Plains may provide protection from cowpea aphid infestations by allowing an increase in beneficial
predator populations before the initiation of alfalfa growth.
'Dona Ana' and 'WL612' alfalfa may have a degree of resistance to cowpea aphid.
INTRODUCTION
In 2001, cowpea aphid (Aphis craccivora) began inflicting
severe damage by attacking alfalfa (Medicago sativa L.) fields
upon spring green-up, causing significant stand loss in the
Southwest, including the Southern High Plains and Southern Great Plains (Ford, 2001; Palumbo and Tickes, 2001;
Summers et al., 2004; personal observation). Cowpea aphid
had been commonly found in alfalfa fields during summer,
but was never considered an economic pest until a strain
developed in California that spread eastward, eventually
infecting fields in western Kentucky in 2006 (Ford, 2001;
Robinson-Avila, 2005; Townsend, 2006). Consequently, little
information is available about its activity in relation to alfalfa
(Palumbo and Tickes, 2001).
Cowpea aphid normally feeds on cowpeas during the summer. Recent infestations damaging alfalfa in winter and early
spring are thought to be a biotype that prefers warm, wet winter and spring periods (NMSU, 2005). Control by predatory
insects very early in the growing season is unlikely because of
low predator populations at that time; thus, insecticide treatment is often necessary (Palumbo and Tickes, 2001). Palumbo and Tickes (2001) found that, while several insecticides
provide adequate control, pyrethroids and carbofuran had
more residual activity on cowpea aphid than did dimethoate.
Currently, no alfalfa varieties have known resistance to cowpea
aphid (Summers et al., 2004), and standardized testing procedures for this pest are not in place (NAA1C, 2004).
Alfalfa varieties are grouped into fall dormancy (FD) categories based on their autumn height. While fall dormancy
is often estimated fairly accurately, values are rounded to the
nearest whole number for marketing purposes. Very dormant
varieties (those ceasing to grow earliest and, therefore, being
shortest) have low numbers, and very nondormant varieties
(those not ceasing to grow, thus being the tallest) have high
numbers (Teuber et ai., 1998). This has been associated with
reduced fall yields, winter survival, and drought tolerance for
dormant varieties and earlier initiation of spring growth, more
rapid regrowth after harvest, and heat tolerance for less dormant varieties (Brummer et al., 2002).
In a study conducted to determine which FD categories
would be best suited to the region of the Southern High
Plains of the USA proximal to the 35th N latitude (Lauriault
et al., 2009), a cowpea aphid infestation during the last year
of the trial provided information that might be of value to
alfalfa breeders and managers and Cooperative Extension Service personnel.
MATERIALS AND METHODS
The main study (Lauriault et al., 2009) was conducted from
1998 to 2001 at the New Mexico State University Agricultural
Science Center at Tucumcari, NM, USA (35.20°N, 103.68°
W; elevation 1247 m). The alfalfa was planted into a Canez
fine sandy loam soil (fine-loamy, mixed, thermic Ustollic Hap-
'Respectively, Forage Agronomist, Agricultural Science Center at Tucumcari (Email: lml.iur<:',iimsu.edu, 650.2 Quay Road AM.5, Tucumcari, NM 88401); Agricultural Biometrician, Agricultural Biometrics Service, Agricultural Experiment Station; and Superintendent, Agricultur.il S< ience t enter at Tucumcari, all of New Mexico
State University.
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