Leafhoppers in Manitoba; Biology, Behaviour and Potential for Vectoring Plant Diseases

Leafhoppers in Manitoba; Biology, Behaviour and Potential for Vectoring Plant
Diseases
John Gavloski, extension entomologist
Manitoba Agriculture, Food and Rural Initiatives, Carman MB, R0G OJO
E-mail: [email protected]
Introduction
High levels of aster yellows were noted in many crops in Manitoba in 2007. Because of this
there is a heightened concern over leafhoppers and aster yellows, and a need to better
understand the vector, the disease, and leafhoppers in general. It is important for agronomist
and farmers to understand that it is specific species of leafhoppers that can transmit aster
yellows, and not leafhoppers in general. This presentation will provide an introduction to
leafhoppers, with a focus on 2 of the potentially economical species in Manitoba. A presentation
to follow will provide more detailed information on aster yellows.
Diversity of Leafhoppers
Leafhoppers belong to a family of insects known as Cicadellidae. There are approximately 350
species of leafhoppers in Manitoba, and it is estimated that there are over 1,200 species of
leafhoppers in Canada (Maw et al. 2000). Only a few of these species can be of economical
concern to agricultural crops. These include the potato leafhopper, Empoasca fabae, which can
cause a condition known as “hopperburn” in some plants, and the aster leafhopper, Macrosteles
quadrilineatus, which can vector a disease known as aster yellows to some plants. Some of the
other species can be abundant at times and in certain habitats, but not be of economical
concern. The grassy areas outside farm fields often contain very high numbers of leafhoppers
that are not of economical concern.
How Leafhoppers Feed
Leafhoppers occur on a diversity of plants, and many are rather specific in their selection of food
plants. They feed by sucking plant sap with their beaks. Leafhopper beaks have 2 channels,
one for shooting in saliva, and another for sucking up food. The injection of saliva into the plants
can be a bigger issue than the amount of plant sap they remove. Some species of leafhoppers,
such as the aster leafhopper, are capable of moving plant pathogens from plant to plant as they
feed. Saliva injected by potato leafhoppers results in abnormal cell growth and interferes with
the transport of fluids in the leaves. This can result in a toxicogenic condition known as
“hopperburn”, which is a wedge-shaped yellowish pattern on the tips of leaves. Both nymphs
and adults of potato leafhoppers are toxicogenic. However, late instar nymphs are reported to
cause greater yield losses than adults due to their increased toxicogenic potential.
Identifying Aster Leafhopper and Potato Leafhopper
Agronomists should familiarize themselves with being able to identify both the potato leafhopper
and the aster leafhopper. Many other species of leafhoppers may be encountered while
scouting crops, and particularly if the grassy field margins are examined, as is done to scout for
grasshoppers early in the season. For proper and economical crop scouting decisions, it is
important to realize that the vast majority of leafhopper species are not potential pests of crops,
and to recognize those that can be.
Potato Leafhoppers are small (average size of adults is about 3 mm long), pale lime-green,
wedge-shaped insects. They have mostly white eyes, with white markings between the eyes.
Nymphs are smaller than adults and lack wings. Potato leafhoppers pass through 5 nymph
stages before becoming adults. First instar nymphs are pale white and change to a yellowish
green after feeding. Eyes are dull red in young nymphs but change to a pearl white by about the
third instar (Fenton and Hartzell 1923 – this reference provides a detailed description of each
stage of the potato leafhopper). Nymphs of potato leafhopper often run sideways when
disturbed; adults will fly or run sideways when disturbed.
Figure 1. Potato leafhopper.
Photo by Tom Murray
Aster Leafhoppers are about 3.5 to 4 mm long as adults, are brownish gray and have some
distinctive markings on their head. This leafhopper is sometimes referred to as the six spotted
leafhopper because on the head, between the compound eyes, there are 6 markings (the top
pair look like spots, and the bottom 2 pair of markings are more like dashes than spots).
Nymphs can range from 0.6 to 3 mm long. They have the same head markings as the adults but
vary in colour from yellow or light brown to a pale greenish-gray.
Figure 2. Adult of aster leafhopper.
Photos by C. Olivier, AAFC-Saskatoon
Figure 3. Nymph of aster leafhopper.
How to Look for Leafhoppers
In most crops, the easiest method of determining whether leafhoppers are present, in relatively
what numbers, and what species, is to use a sweep net. However both rainfall and windy
conditions can affect the levels of some species of leafhoppers caught using sweep net
sampling. In some crops, such as beans, turning over leaves and visually examining them for
the presence of leafhoppers is recommended.
Host Plants
Some species of leafhoppers have a very wide host range, while others are very host specific,
and some are even fussy about what part of the plant they live on.
Potato leafhoppers have a very broad host range. It is capable of reproducing on >200 plant
species in 25 different families. Poos and Wheeler (1943) list some of the host plants from
which potato leafhopper was reared from as including alfalfa, clover, beans, soybean, potato,
strawberry, sunflower, hemp; and trees such as apple, birch, elm, maple, and willow.
Aster Leafhoppers also have a very broad host range, with over 300 species of plants having
been identified as hosts. Some of the plants they feed on include crops such as barley, wheat,
oats, rye, corn, flax, sunflowers, canola, buckwheat, potatoes, carrots, onions, and lettuce; as
well as weeds such as quackgrass, lambs-quarters, and stinkweed. Aster leafhoppers are the
main vector of a disease called aster yellows, which can affect many plants. Because of this, it
is beneficial to know the levels of aster leafhopper, and what percent of the population of
leafhoppers are infected with the phytoplasma that causes aster yellows.
Variations in Aster Leafhoppers and Aster Yellows from Year to Year
The levels of aster leafhopper, and also how many of them carry the aster yellows phytoplasma,
can be quite variable from year to year. In many years, levels are low enough that they are
noticeable only at small levels in field crops. Years when aster yellows does appear in
significant levels in Manitoba, can be followed by years when levels are insignificant. The year
1957 is regarded as one of the worst years for aster yellows in Manitoba. In 1957, an epidemic
of aster yellows was reported in the Great Plains area of North America. The epidemic was
attributed to an abnormally large population of aster leafhoppers, and a high incidence of
viruliferous individuals within the population (Westdal and Richardson, 1966). In Manitoba in
1957, out of 60 flax fields surveyed no fields were seen free of aster yellows, the average
infection rating was slightly over 15%, and the yield loss due to aster yellows was estimated to
be at least 15% (Sackston & Martens, 1957). In the same year, 38 out of 46 sunflower fields
surveyed had symptoms of aster yellows, although differences were noted between varieties,
and aster yellows was noted to have affected 5% of a buckwheat crop at Winkler. Yet the
following year, 1958, aster yellows was not at significant levels, and incidence were so low and
erratic that no results were obtained from sunflower plot experiments on the disease (Sackston
and Martens, 1958).
Where Leafhoppers Come From
How well leafhoppers overwinter on the Canadian prairies varies with the species of leafhopper,
as well as with winter conditions. Some species are quite capable of overwintering on the
Canadian prairies, while for others the populations that are present entirely depend on
populations arriving in spring on winds from the south. For potato leafhopper and aster
leafhopper, the levels that move in on winds from the south each year can be a big determining
factor in how abundant they are.
Potato leafhoppers do not overwinter in Manitoba. Movement of potato leafhoppers into
Manitoba can occur with weather fronts originating over southern states. The arrival of these
migrant leafhoppers varies from year to year depending on factors influencing the development
of spring migrants in the overwintering areas and on the timing of weather systems capable of
transporting the migrants northward (Sidumo et al. 2005).
Although it is possible for aster leafhoppers to overwinter as eggs (Westdal et al., 1961) and
possibly adults (Olfert et al. 2004) in the Canadian prairies, the amount that successfully
overwinter is still quite uncertain. It is generally thought that the majority of the population in the
Canadian prairies is due to leafhoppers arriving on winds form the south. This can explain the
unpredictability of aster leafhopper levels from year to year. Not only is the level that arrives
important, but when they arrive, and how infected the population is with the phytoplasma that is
responsible for aster yellows can also be important determinants of what levels of aster yellows
will be like.
Conclusions
1) Leafhoppers are a very diverse group of insects. Don’t assume all are potential pests of
crops. Learn the distinguishing features of the few that can potentially be pests.
2) Aster leafhopper and potato leafhopper both feed on a large variety of plants.
3) The pest status of potato leafhopper and aster leafhopper will depend heavily on
populations moving into Manitoba from the southern United States, and for aster
leafhopper what percentage of the population is infected with the aster yellows
phytoplasma. This can vary greatly from year to year.
References
Fenton, F.A. & A. Hartzell. 1923. Bionomics and control of the potato leafhopper. Research
Bulletin No. 78, Agricultural Experiment Station, Iowa State College of Agriculture and
Mechanical Arts.
Maw, H.E.L., R.G. Footit, K.G.A. Hamilton, & G.G.E. Scudder. 2000. Checklist of Hemiptera of
Canada and Alaska. NRC Research Press, Ottawa xiii + 220 pp.
Olfert, O.O., G. séguin-Swartz, & C.Y. Olivier. 2004. Pest status of leafhoppers and their impact
on aster yellows disease in canola. Saskatchewan Agriculture Development Fund Final Report,
Project #20000242.
Poos, F.W. & N.H. Wheeler. 1943. Studies on host plants of the leafhoppers of the genus
Empoasca. United States Department of Agriculture Technical Bulletin No. 850. 51pg.
Sackston, W.E. & J.W. Martens. 1957. Canadian Plant Disease Survey. Volume 37.
Sackston, W.E. & J.W. Martens. 1958. Canadian Plant Disease Survey. Volume 38.
Sidumo, A.J., E.J. Shields, & A. Lembo. 2005. Estimating the potato leafhopper Empoasca
fabae (Homoptera: Cicadellidae) overwintering range and spring premigrant development by
using geographic information system. Journal of Economic Entomology: 98: 757-764.
Westdal, P.H. & H.P. Richardson. 1966. Importance of aster yellows virus in field crops in
Manitoba. Canadian Plant Disease Survey. Vol. 46.
Westdal, P.H., C.F. Barrett and H.P. Richardson. 1961. The six-spotted leafhopper, Macrosteles
fascifrons (Stål) and aster yellows in Manitoba. Canadian Journal of Plant Science. 41: 320-331.