Agronomy mimeo #99
Agronomy Mimeo 99
(February 1991)
Potato Leafhopper Control Important in Alfalfa
The potato leafhopper (Empoasca fabae Harris) is the most destructive
insect pest to alfalfa in Maryland. It can be one of the primary limiting
factors to high yields if not properly controlled. It is responsible for
reduced yields, reduced quality (especially lower protein content), and
contributes to reduced longevity of stands. The stress applied to alfalfa
plants by leafhoppers can result in increased root rot incidence and
contribute to stand failure. This can be particularly true in new seedings.
The potato leafhopper is a problem primarily from mid-June through
mid-August. This insect does not overwinter in Maryland. Each spring
potato leafhoppers migrate northward on the wind currents from the Gulf
States, where they breed continuously. Wind currents carry them from the
Gulf States up the Mississippi and Ohio Valleys and disperse them
throughout the eastern United States. They generally arrive in Maryland
in late May or early June. Since they do not survive the winters in
Maryland, we generally escape their damage on first cutting and second
cuttings made by late June. It is the new spring seedings and regrowth
of later second and the third and fourth cuttings that are most severely
damaged. Normally, the population drops off rather sharply after mid-August.
Adult potato leafhoppers are tiny, pale yellowish-green, wedge-shaped
insects, about 1/8-inch long. They are very active, especially when it is
warm, and either jump or fly when disturbed, hence the name leafhoppers.
The best time to find them is between 10 a.m. and 4 p.m. when it is warm
and dry and they have moved to the top of the foliage.
The female lays eggs in the small stems and the leaf veins and
petioles. The eggs hatch in 6 to 9 days into whitish nymphs which soon
turn yellowish-green. In warm weather it takes about three weeks for
development from eggs to adults, so very large populations can build up
in a short period of time. The nymphs resemble adults, but are wingless
and have more pronounced yellow coloring, ranging from bright yellow to
yellowish-green. The nymphs are also very active and move quickly.
A particular characteristic of the nymphs is the ability to walk sideways
or backwards at a rapid pace when disturbed.
Both the adults and nymphs have piercing-sucking mouth parts and feed
by sucking plant juices from the leaves and stems. As they feed they also
secrete a toxic substance into the plants that apparently causes a plugging
of the vascular system. This plugging interferes with normal translocation
of carbohydrates in the plant and can cause discoloration and even dried
out areas at the leaf margins. A typical symptom of potato leafhopper
injury is yellowing followed by bronzing, especially in a triangular or
wedge-shaped area at the tip of the leaflets. On some plants these areas
may dryout and turn brown. This symptom is commonly referred to as
"hopper burn". As the damage increases, the vascular system becomes
plugged and the whole plant develops symptoms. Discoloration spreads
throughout the leaves and the crop is stunted.
Types of losses. Potato leafhopper feeding damage may result in
several types of losses:
1. Stunting of plants. Growth is greatly reduced, sometimes more
than half that of a normal plant. Stunted plants do not recover after
the leafhoppers are eliminated. Once severe yellowing has occurred, the
plants ceases further growth. In this case it is usually better to
harvest the crop and monitor leafhopper populations for possible
insecticide treatment of the regrowth.
Potato leafhopper damage in mid-summer is sometimes mistaken for
drought damage since the stunting and yellowing of the plant can resemble
drought damage. In fact because of the plugging of the vascular system,
damage by leafhoppers is more severe in dry years when the plant is under
moisture stress.
2. Loss in quality. As mentioned earlier the leafhopper injects a
toxin as it feeds, which causes a chemical change within the plant.
Plants produce less protein and more sugars, resulting in a major
reduction in feed value. Protein is the most serious loss in alfalfa due
to potato leafhopper feeding. Purdue University research data suggests
that protein loss occurs very quickly with relatively low insect
populations(Wilson). Five percent loss in protein occurred when the
leafhopper population increased from 5 to 50 leafhoppers per sweep, the
latter being a common population level attained in Indiana when controls
have not been applied.
3. Loss in yield. Loss in dry matter yield occurs, but is usually
less significant than quality loss, according to Wilson.
4. Loss in plant vigor. Leafhoppers can drain alfalfa of its vigor,
resulting in serious carryover effects on later cuttings. These effects
are reflected in:
(a) Slow recovery of regrowth following harvest.
(b) Winterkill, causing stand loss due to plants entering dormancy
in a weakened condition.
(c) Loss in yield the following season. Purdue University research
data showed not only greater vigor, but a production increase of 1/2 ton of hay per acre on the first cutting where leafhoppers had been controlled the previous year.
Losses due to leafhopper damage are frequently attributed to other
factors, particularly dry weather since the effects of leafhopper feeding
are similar to the effects of dry weather. Observing alfalfa fields
around Maryland, particularly in July and August, it appears that many
farmers have not adequately controlled potato leafhoppers, resulting in
yield losses of late second, third and fourth cuttings and possible
carryover effects to the following year.
Wisconsin researchers (Smith and Medler, 1959) investigated the
influence of potato leafhopper on yield and chemical composition of
alfalfa hay grown at two soil fertility levels. Two alfalfa varieties
were evaluated. After establishment in the spring, each variety was
subdivided into two soil fertility plots. One plot received no further
fertilization while the other was top dressed the first fall with 450 lb/A
of 0-10-30 fertilizer and the second fall with 750 lb/A of 0-10-30
fertilizer. Each fertilizer treatment was subdivided into two plots.
One plot was sprayed as needed to control insects while the other was left
unprotected.
Yield and chemical composition data obtained in the third harvest
year are presented in Table 1. Potato leafhopper reduced hay yields of
Vernal 21% and of Narragansett 28% with high soil fertility, and 36% and
48%, respectively, with low fertility. There was only an 11% difference
in hay yields with both varieties due to fertility alone.
Smith and Medler pointed out that the lack of a marked response in the hay
yields to soil fertility alone was due largely to the fact that the
subsoil contained a good supply of P and K. As a result, difference in
hay yields from soil fertility alone did not begin to appear until the
second harvest year of the trial. Note that the two varieties used in
this study were much less productive than varieties used today and the
alfalfa cutting management less intensive than today.
Table 1. Dry matter, crude protein and crude fiber yields and percentage chemical composition
of oven-dry hay from Vernal and Narragansett alfalfa as influenced by soil fertility and insect
prevalence.
_______________________________________________________________________________
Trts* Yield R** Crude Protein Crude fiber
lb/A % % lb/A R % lb/A R
_______________________________________________________________________________
Vernal
HF-Sp 2480 17.5 434 30.8 763
HF-I 1950 21 15.8 309 29 24.4 475 38
LF-Sp 2210 11 19.3 426 2 30.8 681 11
LF-I 1580 36 15.3 242 44 24.5 387 49
Narragansett
HF-Sp 2430 19.1 464 32.4 789
HF-I 1750 28 16.9 295 36 23.7 415 47
LF-Sp 2170 11 20.5 445 4 31.1 675 14
LF-I 1270 48 16.3 207 55 24.0 304 61
_______________________________________________________________________________
*Treatments: HF = high fertility, LF = low fertility,
Sp = sprayed to control insects,
I = insects prevalent
**R = percent reduction from high fertility, sprayed treatment. From: Smith and Medler,
1959.
The percentage crude protein content was reduced considerably by
leafhoppers in both varieties at both fertility levels. The percentage of
fiber was reduced also. Smith and Medler indicated that the reduction in
fiber probably resulted from the inhibition of stem elongation due to
leafhopper damage. Reducing forage fiber content is a desirable
characteristic to improve quality but in this case it is at the expense
of yield and protein.
In general, leafhopper caused larger yield and chemical composition
reductions with low fertility than with high fertility. This data
reinforces the point that obtaining high yields requires an integrated
management program for fertility, pest management and cutting management.
References
Smith, Dale, and J. T. Medler. 1959. Influences of leafhopper on yield and chemical composition
of alfalfa hay. Agron. J. 51:118-119.
Wilson, M. Curtis. Undated. Improving alfalfa forage quality - How to detect and manage the
potato leafhopper problem. Certified Alfalfa Seed Council, Inc. brochure.
Prepared by: Dr. Lester R. Vough Forage Crops Extension Specialist