Many golf course superintendents have three feet of snow on the ground – or more! But believe it or not, spring IS coming and that means that annual bluegrass weevils will be on the move relatively soon.
While I was “out and about” on the conference circuit this winter, I heard a claim that annual bluegrass weevil adults live for two or three years. The person who made the comment was rightfully pointing out that we need to avoid treating any individual insects with a pyrethroid more than once, since repeated exposure of an individual is the fastest way to speed the development of resistance to an insecticide. So the speaker was reminding the group that a fall application of a pyrethroid should not be followed by a spring application of a pyrethroid in that location.
I am not aware of any evidence to support the claim of weevils living longer than a year. Dr. Ben McGraw (Penn State) has conducted studies in the lab and in the field, tracking individual adults from overwintering until they expired. These adults were held in containers that protected them from predation. (The primary purpose of his study was to determine how many eggs a female can produce over time.) He never observed any overwintering adults surviving later than mid to late July of the following year in his studies. Adults that emerge in mid to late summer DO survive the winter, so those individuals might live as much as 9 or 10 months, but there is no evidence that any adults go through two – or more – winters.
In addition Dr. Shaohui Wu, a post-doctoral research associate in Dr. Albrecht Koppenhofer’s lab at Rutgers, has been studying several aspects of weevil reproductive biology. Her studies show that adults do not overwinter with well-developed reproductive systems. It is highly unlikely that adults would develop to maturity in the growing season that they become adults, revert to poorly developed systems for the winter, and become reproductively active again the following year – and continue that process of developing and resorbing for another year or two.
With all of this in mind, perhaps it is wise to review the life cycle of the weevil, as you prepare for this year’s weevils.
In most parts of southern New England (including Connecticut, Rhode Island, and the lower elevations of Massachusetts), the weevil normally completes two or three generations each year. They always spend the winter as adults in areas that provide some protection – in leaf litter, in clumps of higher grass in the rough, along banks of ponds. Once the temperatures begin to warm in the spring, adults start moving from their overwintering sites toward the shorter –mown turf.
Dr. McGraw and I have dissected thousands of spring-collected adults over the past five years, looking at the development of reproductive systems. It turns out that the adults do not mate immediately after becoming active but instead take a week or two for their reproductive systems to mature. (This is one reason why it is a good idea to delay your adulticide application until most of the adults have emerged from their overwintering sites.) And most of them do not feed (there is little or no evidence of fresh food in their guts) until late April or early May in New England.
Adults begin to lay eggs when some signs of spring become apparent, often in late April or early May. Females place two or three eggs at a time inside the leaf sheath, where they are very well protected from weather extremes – and are not accessible to insecticides.
After a week or two the eggs hatch into tiny larvae that remain inside the stem, where they feed for a week or 10 days. They molt to a second instar and continue feeding inside the stem for another week or so. These larvae then molt to the third instar, emerge from the plant, and make their way down toward the crown of the plant, where they complete two more stages, each lasting a week to 10 days. The fifth (and largest) instar causes significant damage in the crowns. By the early to middle part of June, most larvae will complete their development and pupate in the soil for about a week. We often see older adults (black and more shiny than their younger compatriots, because the scales have rubbed off) dead or dying in short turf in mid to late June.
New young adults (often reddish-brown when they first emerge) will start to show up in mid to late June. They mate, lay eggs, and repeat the process described above. But because summer temperatures are warmer, each stage will develop more quickly. For example it might take only six or seven days to complete each larval stage. So it is possible to see new adults by early to mid August, and THEY can have time to lay eggs for a third generation.
It is always very challenging to determine how many generations occur in a given location in a given year, and in fact it is likely that some larvae that we find in late summer are indeed from a “third” generation while others might have arisen from adults that were a little slower to develop in the spring. In other words, many times in southern New England we have a mixed population – some completing three generations and some completing only two generations.
That generational overlap makes it very difficult to achieve good control of weevil populations in the summer months, so it is really important to knock down the first generation as much as possible. We will talk about that when we get closer to the time to make the first applications.
Submitted by: Dr. Pat Vittum