Idaho Lygus Parasites

Lygus Parasitoid Progress Report for Summer 1999
Prepared by Ezra Colgrove and Ron Bitner

Sample Locations

At the beginning of this project, five alfalfa seed fields at different locations in the Treasure Valley of Idaho were selected as sites for lygus nymph collection. It was planned for lygus to be collected from these fields once a week and the lygus would be dissected for the presence or absence of internal parasitoid larvae. It was soon apparent that it would not be possible to collect lygus from these fields each week because the lygus numbers were not high enough to collect a sufficient amount of lygus due to pesticide applications. Instead, hay fields were chosen as sites for weekly lygus collection and lygus were also collected from alfalfa seed fields when lygus numbers were around one to two or more lygus 3rd though 5th instars per 180 degree sweep of a sweep net. Due to changes that occurred to both hay fields and seed fields throughout the season (hay fields being harvested or seed fields being sprayed), it was not possible to collect lygus from any site every week throughout the collection period. Some weeks it was not possible to collect enough 3rd-5th instar lygus because at that period in the lygus population lifecycle there were no lygus of the above mentioned instars present.

Parasite Sampling Technique

The lygus collection process was quite simple. The edges of a field were swept with a sweep net and 3rd-5th instar lygus were collected from the net with an aspirator. Lygus were collected from the net in an unbiased manner. Collected lygus were transferred to a container, which was then labeled and placed in a cooler. When all the lygus were collected for the day, the containers were transferred to a freezer and the lygus were killed through freezing.
When it came time for the lygus to be dissected, the lygus were transferred to a dish containing water with a small amount of dish soap (to break surface tension). Dissection was performed under a dissecting microscope with two pairs of forceps. With once pair of forceps, the lygus was grasped by the distal two or three abdominal segments. The other pair of forceps was used to grasp the lygus by the thorax. The lygus was then carefully pulled apart (being separated between the first and second pair of legs). If the lygus contained a large (older) parasitoid larva (Figure 1), the larva would be forced out by pressure that had built up within the lygus body cavity. If no parasitoid emerged from the lygus in this manner, the lygus would be carefully stroked in the direction from abdomen to thorax. The number of parasitized lygus was kept track of and recorded, as was the total number of lygus dissected. All parasitoids collected from the lygus were preserved in vials of alcohol and labeled.

Results

As can be seen in the data, there were many more parasitized lygus collected in the later part of the collection period than there were early on.(Table 2: Field 15; 39 out of 52 Lygus bugs sampled (75%) were parasitized). This could be that the parasitoids did not show up until later in the season, but it could also be that since the sites of collection were changed through the collection period, it just seems that there may have been more parasitoid activity later on. There was a tendency to visit those sites from week to week that had greater parasitoid activity. It was also the case that sites of high parasitoid activity were not found until later in the collection period.
The data collected gives a good starting for further study. It would help to have some collection sites that would be undisturbed throughout the season so that weekly collection would not be hampered. It would also be of interest to collect from sites in the Middleton area (an area of great parasitoid activity) early on in the season, then it could be known if the parasitoids are as active early in the season as they are later in the season. Another possible study of interest would be to discover the impact the parasitoids make on the lygus population as a whole. This last season showed relatively little lygus activity early on but the cause of this is not known.

Rearing

Besides collecting and dissecting lygus to find parasitoids, there were also attempts to rear parasitoids out of the lygus nymphs. The first attempt of this was unsuccessful. Lygus 3rd-5th's were collected from two sites: Dan Church "1" (6/13) and Hayfield by Shoemaker Orchard on Lakeshore Drive (6/15). Two gallon-size plastic boxes were obtained, holes were cut in the lid and nylon mesh covered the holes. Fifty to 100 lygus were placed in these boxes with fresh alfalfa foliage and kept at room temperature and humidity. No parasitoids were reared out from this attempt. The next attempt utilized small cup-sized plastic containers with screw-top lids (Takasugi "Egurrola 4" (7/21) and Hayfield by Shoemaker Orchard (7/23) ). About a dozen lygus were placed in a container with a piece of paper towel and some alfalfa foliage. Parasitoid larvae would emerge from the lygus, but would not pupate. Also in this attempt, a layer of dirt was added to the bottom of three of the plastic cup containers, along with holes in the top of the jars covered with nylon mesh. This time parasitoids emerged from the lygus, burrowed to the bottom of the soil, and made silken cocoons (Figures 2 & 3). From two of the jars the pupae were retrieved and placed in small vials (8/11) while the pupae were left in the soil in the third jar. As of yet (8/27) the parasitoids have not emerged as adults. If the pupae never emerge, one possible reason could be a needed period of vernilization. There may also be other necessary environmental requirement not yet known. A requirement now known to be needed is a burrowing substrate. Possible related studies of interest would be how deep the parasitoid larvae will burrow in a natural setting, when the adult parasitoids emerge in the spring (if they overwinter underground) and how many generations there are per year.

Discussion

The objective of this project was to do a broader survey for the native parasite (now identified as Peristenus howardi,W.H.Day et.al, 1999) that was found near Parma in 1997 and 1998. The parasite was discovered in 12 out the 18 sites surveyed in 1999. Adults were not captured or reared out so a positive identification to species was not possible. The fact that the parasite was so common (67% of the fields sampled) and in quite high numbers in some locations (Field 15, 75% of lygus sampled contained the parasite) has quite important implications for this little wasp. More needs to be discovered about it's biology and potential as a biological control agent for Lygus. Lygus numbers were low in many fields in the summer of 1999 and is documented in the Pioneer field scout reports from their fields. Was part of the reason for these lower counts due to parasitism from this wasp. Further work is required to answer this question.

Day, W. H., C.R. Baird and S.R. Shaw. 1999. New, Native Species of Peristenus (Hymenoptera: Braconidae) Parasizing Lygus hesperus (Hemiptera: Miridae) in Idaho: Biology, Importance and Description. Annals of the Entomological Society of America. 92(3): 371-375.