Sunflower moth: Behavior, scouting, and management
The sunflower moth, Homoeosoma ellectellum, is the most serious pest of commercial sunflowers in the central Great Plains. It is well adapted to exploiting commercial sunflowers where large fields of plants develop and flower all at the same time. The oversized flowers of commercial varieties may host hundreds of larvae, and their feeding damage permits infection by Rhizopus, a fungus that requires physical injuries to infect the plant. Without timely treatment, moth damage followed by Rhizopus infection can result in total yield loss over a large acreage. Losses from this pest, commonly known as the head moth, have deterred many growers from incorporating sunflowers into rainfed crop rotations where the plant is a natural fit.
Figure 1. Sunflower moth adult. Photo by J.P Michaud, K-State Research and Extension.
Figure 2. Sunflower moth larvae on exposed flower face. Photo by J.P Michaud, K-State Research and Extension.
Life history and behavior
The sunflower moth is a migratory pest that breeds year-round in northern Mexico and moves northward annually, usually increasing in numbers with successive generations. Migration is facilitated by seasonal weather patterns that prevail across the southern Great Plains during midsummer and generate a recurrent southerly wind pattern known as the ‘low level jet’ that is ideal for transporting airborne insects over long distances. The reproductive success of earlier generations probably affects the numbers of moths in each annual migration. Under warm conditions, the moths can complete a generation in 30 days. Kansas can experience secondary generations of local origin following the initial migration, especially in years when wild sunflowers are locally abundant. Early blooming fields where moths are not controlled can also serve as a source of infestation for later planted ones.
The large blooms of cultivated sunflowers represent a huge food supply compared to wild composite flowers and are capable of supporting the development of many larvae, while at the same time providing a refuge from parasitism. Research has shown that most wasps are unable to parasitize moth larvae within such large flowers and soon abandon them, probably because their ovipositors are not long enough to reach them deep within the receptacle. Certain parasitic flies of the family Tachinidae are slightly more successful because they deposit live larvae able to penetrate the flower in search of a host. Although biological control may contribute to suppression of sunflower moth in wild host plants, parasites and predators cannot prevent economically damaging infestations in commercial fields.
Successful management of sunflower moth hinges on rigorous monitoring of individual fields and prompt insecticide application once threshold numbers are detected. Conventionally, the goal has been to kill adults and early instar larvae before physical injury to the flower occurs that can permit fungal infection. Recently, new control tactics became possible with diamide insecticides (chlorantraniliprole, flubendiamide); these have some systemic activity within the flower and thus a greater duration of efficacy. However, these are not contact insecticides and require consumption by larvae; they will not kill adult moths, but have the advantage of being safer for pollinators.
Fortunately, early instar larvae feed exclusively on pollen and remain vulnerable to contact insecticides for several days before they burrow into the receptacle and begin damaging seeds. Later instar larvae feed more aggressively and are less vulnerable to contact insecticides deep within the receptacle. Rhizopus fungus can quickly rot the entire head and will invade vascular tissues to become systemic within upper plant parts, inhibiting seed fill.
Early-planted fields typically suffer the most damage from migratory flights of sunflower moth in Kansas. Fields planted in early July or those that bloom after August 10 are less likely to develop economic infestations. They may still be affected by second-generation moths that emerge from patches of wildflowers or adjacent early-planted fields where the pest has not been adequately controlled. Geographically, average moth pressure tends to decrease from eastern to western Kansas, reflecting the prevailing paths of summer winds.
Because migratory moths can appear in large numbers virtually overnight, scouting should begin as soon as the first flowers open (R5.1) and continue every 2 - 3 days until pollen shed is complete in a majority of plants. Because moth migrations are spotty and each field has its own unique developmental schedule, it is not advisable for growers to follow the management decisions of neighbors; there is no substitute for scouting individual fields. Fields that have been sprayed should be revisited to determine the efficacy of treatment as soon as the re-entry interval expires. The treatment threshold for application of contact insecticides is 1 - 2 moths per 5 plants. Projected crop value can be used to elect the low or high end of this range.
Moths usually rest underneath leaves in the daytime and fly up when disturbed. If moths are abundant, they will be apparent during the day. If marginal numbers are present, scouting should be conducted an hour after sunset when moth activity on flowers reaches its peak. Use a flashlight to count the moths on a series of 20 flower faces in at least five locations in the field.
Historically, sunflower moth activity has also been monitored with commercially available pheromone traps that attract and capture male moths. Traps are best placed on T-posts above canopy level at least 10 rows into the field on north and south sides, with at least four traps per field. Trapping should begin as plants enter the R-5.1 stage (ray petal emergence) and trap catches monitored daily through the R-5.8 stage (80 percent pollen shed). Insecticide applications should be considered whenever pheromone traps average four moths per trap per day. If trap catches average fewer than four per day, field scouting is recommended to determine whether the action threshold has been reached. Trap catches averaging less than one per day usually have resulted in noneconomic infestations. However, it is questionable whether the information provided by trapping is sufficiently useful to justify the labor it requires, especially when certain outcomes dictate manual scouting before a decision can be made. Strong winds can lead to ‘passive catch’ of other moth species that may be difficult to distinguish from sunflower moths once they are stuck to glue in the trap. In addition, doubts have been raised about the persistence of the pheromone lure under Kansas field conditions that typically involve high temperatures and strong winds.
For more information on scouting and monitoring, see the most recent edition of the K-State Research and Extension publication, Sunflower Insect Management, MF814: http://www.ksre.ksu.edu/bookstore/pubs/MF814.pdf
J.P. Michaud, Professor of Entomology, Agricultural Research Center-Hays
(Note: This article is an updated excerpt from K-State Research and Extension publication MF3108, Sunflower Moth, MF3108: http://www.ksre.ksu.edu/bookstore/pubs/MF3108.pdf)