Trefoil Seed Chalcid

Agdex#:	122/612
Publication Date:	01/91
Order#:	91-016
Last Reviewed:	01/91
History:
Written by:	C.R. Ellis - University of Guelph

Table of Contents

1. Biology
2. Damage
3. Control

Biology

The trefoil seed chalcid is an important pest of trefoil that is grown for seed production. Chalcids also attack clover and alfalfa seeds, but these chalcids are not the same species.

Figure 1. Biology of chalcids:
A, exits holes made by emerging chalcids;
B, a chalcid wasp on a trefoil pod (Curtesy of Steve Peterson);
C, roadside trefoil which is a source of infestations.

The small female wasps (Figure IB) lay their eggs in the young green seeds within the developing trefoil pods. They hatch into white legless grubs that feed inside the seeds. They moult four times and hollow out the seeds before they complete their development. They pan the winter in the seeds which are scattered on the ground. However, some of the wasps emerge after mid August (Figure IA shows the typical exit holes in in the the pods). These adults that emerge in August lay eggs for a small partial second generation.

Chalcids emerge later in northern Ontario than in the south.In both areas, first emergence occurs just before the trefoil begins to bloom. The adults continue to emerge from he seeds in which they have overwintered over the next month. Because of this long period of emergence, and because the adults live for a month or more, populations increase progressively through the summer and peak about mid August in both southern and northern Ontario (Figure 2).

Figure 2. Seed chalcids become more numerous in both southern and northern Ontario as the season progresses.

Trefoil flowers over a long period of time, and sets pods for up to 7 weeks. Therefore, green pods are still available in mid August when some first generation adults began to emerge from brown pods. This partial second generation is he reason for the peak in chalcid numbers in late August.

Damage

The trefoil seed chalcid occurs wherever trefoil seed are produced in Ontario. Infestation range from I to 45 % in commercial fields. However, an infestation level of about 20% is average in older Ontario felds. Trefoil on roadsides and in unmanaged pastures (Figure IC) can have 50%. of the seeds infested at certain times.

In general, infestations are also higher in older fields. This trend has also been noted in other countries. This build-up occurs because infestations in new fields begin only with chalcids that spread in from surrounding areas. During the growing season, however, some infested seeds are lost in the field each year because of pod shattering. Chalcids from these seeds emerge the following year to contribute to a yearly infestation cycle within the field.

Examining samples seed after cleaning underestimates the severity of the chalcid problem. Seeds hollowed out by chalcids often break up and are lost during cleaning. They are also somewhat lighter and many of them are removed during seed cleaning.

Often chalcids emerge from seed that is in storage. These wasps cause great concern because of fear that hey will lay eggs and infest more of the seed. However, chalcids cannot infest stored seeds. They are only able to lay their eggs in soft developing seeds in the field. Infested seeds in storage must be recleaned to remove the chalcids and damaged seeds, but there is no need for a chemical treatment.

Control

Two species of parasites attack chalcids in Ontario and kill about 20 % of them. However, chalcid populations remain high in spite of these parasites. Also, an infested seed is destroyed whether or not the chalcid inside is parasitized.

Trefoil blooms over a long time so that flowers, which require pollination, occur at the same time as young pods which are attacked by the chalcids. There are no insecticides registered against this pest in Canada, or, indeed in North America. Attempts to control chalcid infestations by knocking down the chalcid populations with a spray just before bloom were unsuccessful.

The only alternative for controlling this pest is to use cultural controls. Cultural controls are used throughout the world wherever trefoil seeds are grown. The following practices will result in less infested seeds.

1. Harvest the seed crop from the first cut rather than from the re growth because populations of chalcids are higher later in the season (Figure 2). In an experiment in four fields in southern Ontario, about 3.8 % of seed from the first crop were infested, but this level increased about five-fold to 19.1% on the re-growth. However, there may be fewer pollinators in the spring and his may lead to poorer seed-set. Thus, it may be necessary to enhance pollination through provision of bee hives, or by planting pure stand trefoil where pollinators work more easily.
2. Isolate seed fields from other trefoil. Trefoil that grows wild on road-sides and in wastelands (Figure 1C) is heavily infested with chalcids. Many chalcids spread from these areas to infest nearby seed fields (Figure 3).
   Because chalcids move between fields, it also helps to isolate new fields from old ones.

Figure 3. Chalcids are more numerous in areas closest to old infested fields or unmanaged trefoil.

3. Cut or destroy unmanaged trefoil growing near seed fields. This practice may be difficult because such trefoil is widespread in the same areas where it is grown for seed production. Also, trefoil sets pods close to the ground so many pods occur even in grazed pastures. However, it may be possible to mow pastures to prevent pods from maturing.
   In any case, the heaviest infestations of chalcids in new seed fields in Ontario are in areas adjacent to old infested trefoil fields (Figure 3) or trefoil growing along roadsides or in unmanaged pastures. Anything that can be done to eliminate these sources of infestation will be worthwhile. Authorities in other countries recommend a 1.5 km separation between seed fields and all other trefoi1. If trefoil in these zones cannot be destroyed, it should be cut regularly to prevent seeds from maturing.
4. Infestation becomes more severe the longer that fields stay in seed production. Old fields usually have more chalcids than newly seeded fields. This is because pods shatter in the field each year, and some of this last seed contains chalcids. These chalcids only have to travel a few inches to in test seed when they emerge in the held the following year. This within-field chalcid cycle can be broken in good fields by harvesting only hay for one year, instead of the usual hay plus seed crop. If hay is cut before pods are able to mature, there will be no seeds in which die pest can develop in the field, and the cycle of chalcids within the field will be broken.
5. Never leave a seed crop unharvested in the field. Poor weather can result in a poor seed crop, or can prevent harvesting it altogether. During these bad years the potential for seed production is so low hat seed fields may be abandoned Such fields will have more severe chalcid problems the following year. This is because all the seeds, sound and infested, fall to the ground, and the wasps are ready to infest the crop the following year. If a crop is not harvested for seed for any reason such as poor weather, consider cutting and destroy it to prevent heavy infestations the next year. If this is not possible, use the field for hay the following year.
6. Harvest seeds under the right conditions. The more pod-shattering occurs during harvest he more infested seeds get left in the field to increase problems on the following crop. Harvesting early gives rise to a high proportion of immature seeds which fail to germinate. In contrast, harvesting late misses the early maturing seed which falls to the ground in the field. The indeterminate growth habit of trefoil means that pods mature non-uniformly in the field. Thus, the ideal time for harvesting is difficult to determine, and of course, the problem is compounded by the weather. Elsewhere, experimented have been done with desiccants to facilitate uniform drying of pods and to reduce their shattering. This would not only increase seed yields but would also reduce he amount of infested seeds dropped in the field to cause problems the following year.
7. Do not locate seed fields next to pastures. Trefoil pastures re-seed themselves freely even when closely grated. This means that despite grazing, some pods manage to grow to maturity and constitute sites in which chalcids survive to infest nearby seed fields the following year. Because grazing does not prevent pod-set, trefoil pastures should not be located near seed fields, or they should be cut to prevent pods from maturing.
8. Destroy chaff and screenings. They invariably contain chalcids and therefore should be burned or buried. Heaping screenings in a dump to decompose does not kill chalcids near the surface. These chalcids add to the problem in areas near the dump-site.
9. Use Clean Seeds. Chalcid problems may start in fields during seeding if the seed is infested. Even samples of clean seed obtained from seed cleaners in southern Ontario contained some chalcids. Thus, to prevent "recycling" chalcids, start with clean seeds.
10. Uniform harvest dates. Cutting trefoil bay forces the chalcids in the field to move elsewhere or die. There is less chance of them finding flowers and surviving if most of the hay fields in an area are cut at about the same time. Effective management of chalcids can only be achieved if seed production is regarded as an important crop and not just a sideline to hay. Managing the crop for seeds means separating it from other trefoil which is heavily infested with chalcids, and using other cultural controls, as suggested above, to prevent a build-up of chalcids in the seed fields. Research done elsewhere has shown that burning the fields in the spring is not effective against chalcids.