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

Transcription

1 Leafhoppers in Manitoba; Biology, Behaviour and Potential for Vectoring Plant Diseases Introduction John Gavloski, extension entomologist Manitoba Agriculture, Food and Rural Initiatives, Carman MB, R0G OJO High levels of aster yellows were noted in many crops in Manitoba in 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.

2 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.

3 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

4 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 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 Checklist of Hemiptera of Canada and Alaska. NRC Research Press, Ottawa xiii pp. Olfert, O.O., G. séguin-swartz, & C.Y. Olivier Pest status of leafhoppers and their impact on aster yellows disease in canola. Saskatchewan Agriculture Development Fund Final Report, Project # Poos, F.W. & N.H. Wheeler Studies on host plants of the leafhoppers of the genus Empoasca. United States Department of Agriculture Technical Bulletin No pg. Sackston, W.E. & J.W. Martens Canadian Plant Disease Survey. Volume 37. Sackston, W.E. & J.W. Martens Canadian Plant Disease Survey. Volume 38.

5 Sidumo, A.J., E.J. Shields, & A. Lembo Estimating the potato leafhopper Empoasca fabae (Homoptera: Cicadellidae) overwintering range and spring premigrant development by using geographic information system. Journal of Economic Entomology: 98: Westdal, P.H. & H.P. Richardson 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 The six-spotted leafhopper, Macrosteles fascifrons (Stål) and aster yellows in Manitoba. Canadian Journal of Plant Science. 41: