Potato Leafhopper Damage to Alfalfa
by D.B. Hogg1, J.L. Wedbery1, D.J. Undersander2, and K.G. Silveira2
The potato leafhopper (PLH) is perhaps the most serious insect pest of alfalfa in Wisconsin and the Upper Midwest. This leafhopper is a native species but is not able to survive northern winters, and adults migrate into the Midwest each spring. Leafhopper populations frequently increase to damaging levels on the second and subsequent alfalfa crops. Until recently, crop scouting and insecticide application when warranted was the only effective means of PLH management. However, development and marketing of glandular haired alfalfa varieties appears to be changing the way we manage this pest and may ultimately alter the very pest status of the insect on alfalfa.
The adult PLH is a small (1/8-inch-long) green to greenish-yellow, wedge shaped insect. There are five nymphal stages, with the fourth and fifth stages having noticeable wing pads. The first three stages lack wing pads and tend to scurry up and down the plant stem when disturbed, whereas stages 4 & 5 tend to “hop” when disturbed. PLH eggs are deposited below the surface of alfalfa stems and petioles by means of the female’s sword-like ovipositor.
Biology and Seasonal History
Three life history characteristics of the PLH are central to its pest status. First, the leafhopper undergoes an annual migration northward from its overwintering range in the southern U.S., typically reaching Wisconsin in mid-May. Adults can fly hundreds of miles nonstop during this migration, which is wind-assisted, and adult potato leafhoppers also tend to be very dispersive locally throughout the summer. There is also thought to be a return migration back south by PLH adults during the late summer, though direct evidence for this phenomenon is lacking.
Second, the PLH feeds on and is capable of reproducing on many different plants, both wild and cultivated. In addition to alfalfa, crops such as potatoes, soybean and other legumes are at risk (though pubescence on soybean leaves confers resistance to PLH feeding). In addition, ornamental plants and deciduous trees such as maples also serve as host plants for the PLH. In fact, current thinking is that early in the season the migrant PLH adults utilize the tender young leaves of trees such as maples and locusts for feeding and reproduction, and it is largely the new generation of adults produced on these trees that move into alfalfa fields. An important exception to the wide range of host plants utilized by the potato leafhopper is the grass family. PLH generally do not feed, and have never been reported to reproduce, on grasses.
The third important characteristic is the explosive population growth potential of the PLH. The population starts off slow after spring arrival of migrants. PLH adults tend to lay only a few eggs, typically 3 to 7, per day, but the adult females can live and reproduce for 30 or more days and thus can deposit over 200 eggs. Egg and nymph development takes about 9 and 13 days, respectively, so there is a delay of a little over three weeks before new adults appear. However, once these new adults are produced and the leafhopper generations overlap so that all leafhopper stages are present, population growth shifts into high gear. This typically occurs late June or early July in Wisconsin. For example, under normal summer temperatures the amount of time necessary for potato leafhopper populations with overlapping generations to double in size is less than 10 days!
These three life history characteristics – movement, host range and population growth – have implications for the pest status of PLH in alfalfa. First, this leafhopper rarely if ever causes economic injury to the first alfalfa crop in established fields. As mentioned, most of the spring migrants are attracted to trees for feeding and reproduction, and it is primarily the offspring of the migrants that disperse to alfalfa. Furthermore, the cycle of alfalfa harvests slows down leafhopper population growth. Alfalfa fields are cut every 35-40 days during the growing season, and thus the leafhopper population growth cycle is interrupted. When a field is cut, the eggs are removed (in the plant tissue) with the hay and most of the nymphs die from either desiccation or starvation. However, PLH adults can survive by dispersing and locating favorable habitats and host plants. Then as alfalfa fields regrow, PLH adults disperse back in to feed and lay eggs. At what point does the potato leafhopper become an economic pest in established alfalfa stands? Somewhat ironically, established stands are not very conducive to PLH population growth, in fact it can be argued that the leafhopper is “wasting” much of its reproductive effort in alfalfa, since eggs and nymphs are destroyed through cutting and harvesting.
Two situations lead to economic leafhopper infestations. First, when PLH populations are very high the crop can be overwhelmed with adults dispersing into a field and laying eggs early in the alfalfa growth cycle, thus leading to a buildup of not only adults but also nymphs before the crop is cut. Second, there are situations under which nymphs can survive the cutting and harvesting procedures, and when this happens in large numbers the nymphs feeding on the initial crop regrowth can have detrimental and sometimes devastating effects on crop development.
An important exception to the scenario with established alfalfa stands involves spring alfalfa seedings, whether with a companion crop such as oats or established directly. Because new seedings must grow 60 or more days before a first cutting is advised, they are exposed to continuous potato leafhopper population buildup and explosive growth in a way established stands are not. We have seen situations in which a new seeding not cut in a timely way was estimated to produce over 1 million new leafhopper adults per acre per day by the time the field was in full bloom!
A curious and poorly understood aspect of potato leafhopper population dynamics is that late in the season, typically mid-August in Wisconsin, leafhopper population growth stops and numbers level off or even decline. Various theories as to why this occurs have been offered. For example, natural enemy effects, particularly a fungal pathogen that produces disease in potato leafhopper nymphs and adults and a parasitic wasp that attacks the leafhopper eggs, start taking their toll. Also, there is circumstantial evidence that late in the season the leafhoppers cease reproducing and undergo a “return” migration to their overwintering habitats in the south. Regardless of the cause, the late season leafhopper decline tends to be predictable and can be used in pest management decision making.
Potato leafhoppers have piercing-sucking mouthparts, thus they obtain their food by removing sap from plant tissues. However, the primary injury to alfalfa from PLH feeding occurs not from sap removal but from effects on the vascular (phloem) tissue. These effects are not completely understood, but they seem to be partly mechanical but mostly from toxins (digestive enzymes) that are injected by the leafhopper into the plant during feeding. The result is that phloem elements are constricted or blocked so that the carbohydrates produced through photosynthesis cannot be translocated from the leaves to the roots. The telltale sign of leafhopper injury to alfalfa is “hopperburn”, a V-shaped yellowing of the leaf. Feeding injury by potato leafhopper can result in loss of yield and forage quality in the current crop. In addition, the effects of leafhopper injury can carry over to subsequent alfalfa crops, both in terms of yield and alfalfa stand persistence.
Scouting and Economic Thresholds
Effective management of PLH populations depends on sound scouting procedures using a sweep net. The leafhopper is small and very flighty, and it is impossible to see or count adults without the aid of a net. Leafhopper nymphs can be seen if plants are examined, but a sweep net is the more efficient way to sample. Economic loss will already have occurred if a grower waits to act when hopperburn appears in the crop.
Economic thresholds, based on sweep net counts and considering yield/quality losses for the current alfalfa crop, have been developed for PLH. These thresholds are dynamic in that they increase with crop height, and in some cases thresholds have been refined to incorporate differences in forage. Carryover effects of leafhopper feeding injury on yield or stand persistence are not represented explicitly in these thresholds.
The life history characteristics of the potato leafhopper discussed earlier dictate in large measure the approach to be taken for management of this pest in alfalfa. This insect is an active flyer and good colonizer that can utilize many different host plants and has potential for rapid population growth, in short what ecologists would term an “r-strategist”. Thus, PLH population dynamics are likely to be governed more by events outside alfalfa fields than by our actions within an alfalfa field. Also, pests of this type are not generally regulated very well by natural enemies – an r-strategist pest tends to stay “a step ahead” of its enemies. Thus, biological control will probably not provide effective or economical control of PLH populations in alfalfa. The most effective management strategy is to limit as much as possible leafhopper population buildup and reproduction in alfalfa, and take therapeutic action when populations exceed the economic threshold. Discussion of specific tactics follow.
Potato leafhopper is attacked by a variety of natural enemies, including generalist predators (lady beetles, damsel bugs, minute pirate bugs), egg parasitoids (Anagrus spp.) in the family Mymaridae, and a fungal pathogen (Erynia radicans). However, these cannot be counted on individually or collectively for economical control of PLH populations in alfalfa – the natural enemies either have insufficient impact or exert their impact late in the season after PLH populations have peaked and damage has been done. Of the natural enemies, the fungal pathogen has the greatest potential for leafhopper control through disease epizootics. If incipient epizootics can be recognized, unnecessary insecticide applications can be avoided.
Because alfalfa cutting and harvesting is so disruptive to PLH population growth, it was once thought that the leafhopper could be managed solely by manipulating harvest frequency and/or timing. Timely harvest (late bud to early bloom) is certainly important to optimize forage quality as well as to limit PLH population buildup. However, manipulation of harvest frequency cannot generally be used to manage PLH below the economic threshold, particularly if a grower wants to optimize yield/quality of the forage and does not want to jeopardize stand longevity through too frequent cutting.
A second approach to cultural control of potato leafhopper that has received considerable attention recently is the use of alfalfa/grass mixtures. Grasses are not utilized as host plants by the PLH, and it has been shown that mixed stands of some grass species with alfalfa can serve as a deterrent to leafhopper colonization and population buildup. However, the tradeoff is that the quality of the forage from mixed stands may be compromised compared with pure alfalfa stands. The principle of grass as an inhibitor has been shown to extend to the long-standing practice of using a small grain, most commonly oats, as a companion crop to establish alfalfa. However, despite the possible short-term benefits of a grass companion crop, PLH can still build up to damaging levels in alfalfa under the grain crop, and companion cropping alone should not be relied upon to manage PLH populations in spring seedings.
Insecticides are the only effective therapeutic tool for reducing potato leafhopper populations that have exceeded the economic threshold. Insecticide application is recommended only when combined with scouting to detect leafhopper buildup in the alfalfa crop and prevent populations from reaching economically damaging levels.
Until very recently, plant resistance could offer little for potato leafhopper management in alfalfa. However, developments in alfalfa breeding for resistance have occurred that may revolutionize PLH management in alfalfa, namely glandular haired (GH) alfalfa varieties with resistance to PLH. The glandular hair trait was obtained from uncultivated relatives of alfalfa and through conventional breeding methods was incorporated into alfalfa lines possessing acceptable agronomic characteristics. Although the presence of glandular hairs on the leaves and stems is thought to be responsible for PLH resistance, the mechanism by which this occurs is poorly understood. In fact, evidence exists with GH alfalfa for all three main types of crop resistance to insects to be operating: antibiosis, nonpreference, and tolerance.
Starting in 1997, the year GH varieties first became commercially available, we have conducted variety trials with PLH resistant material at the UW Lancaster Agricultural Research Station. Each year (1997, 1998, 1999) we established a new seeding with commercial and experimental GH varieties, and one or more susceptible (non-hairy) checks. Yields were recorded at each harvest. In addition, PLH nymph counts (using a “pan sweep” method), hopperburn ratings (visual assessment of percent yellowing), and crop heights were taken at irregular intervals.
PLH infestations at Lancaster were light or fleeting in 1997 & 98, and thus results from those two growing seasons were not particularly interesting. However, PLH infestations and severity in 1999 were at a ten-year high, and useful results were obtained for the 1998 and 1999 seedings. Briefly, strong responses were found for yield, PLH nymph numbers, hopperburn and crop stunting. However, yield response to PLH injury in GH varieties was not always related in expected or predictable ways to the other three indicators of leafhopper severity. These relationships have significance for the mechanism(s) of resistance and implications for PLH management. Data from these trials and statistical results (analyses of variance) can be found at http://www.uwex.edu/ces/forage/.
1Department of Entomology, University of Wisconsin-Madison
2Department of Agronomy, University of Wisconsin-Madison