• Introduction
• Steps for sampling
• Interpreting sampling results
• More on monitoring

Introduction to monitoring

The presence of Trichogramma will leave some tell tale signs like "black eggs" and lower than expected larvae counts considering the egg counts.

These signs give a general indication but for more accurate estimates of parasitism it is recommended that egg samples be taken and grown out to see if a grub or wasps emerges. Further, just relying on the numbers of "black eggs" in the crop can be misleading. Parasitised eggs may fall off in wind or rain but still produce wasps. Its not uncommon to measure high levels of parasitism in crops where few "black" eggs are observed.

Sampling eggs may seem awkward at first but you soon get the hang of it.
The information it provides can save much work and money.

Percentage parasitism is determined by collecting the target host eggs, placing them singly into cells of a multi cell tray and observing them to see whether a grub or wasps emerge.

By doing this weekly during pressure periods a history is developed and trends can be observed.

The following implements make the job easy:

• Single hole paper punch
• Long fine tipped tweezers
• 96 cell microtitre tray
• very clear broad sticky tape
• masking tape for recording collection data on the tray
• fine needle ( acupuncture needle or "Ultra fine" gauge 29 insulin needle)
• good quality 10 x hand lens,
• felt tipped pen.

Take an Esky with you to store the trays of eggs.
This ensures they are not left in a hot car or in direct sun which will kill the eggs.

The steps for sampling Helicoverpa are as follows:

• Collect at least 20 eggs, from across each block. Collect 5 or so eggs in an area but not from the same plant then move to a different spot and so on.
  
• Select "brown ring stage" eggs if possible (right), these are usually two to three days old. Pure white eggs may have just been laid and may not have had time to be parasitised. (If few brown eggs are present then collect white eggs but make a note of this at collection and put them in a separate row in the tray).
  
• Eggs are collected in the field by punching out discs of leaf containing the egg with a single hole paper punch (right). The discs are placed into a multi cell tray or similar. Alternatively, sections of leaf can be collected in the field and the eggs removed later with a fine paint brush.
  
• Cover the tray with strips of clear, wide sticky tape. Importantly, using a very fine needle make several very small holes in the tape over each cell. This will allow the moisture from the leaf to escape and prevent mould. Label the tray with date, site, etc. Keep in a warm place (around 25 to 30 deg C if possible) but out of direct sun.
  
• Most of the eggs that are not parasitised will darken and produce a small caterpillar in about 3 days (depending on temperature). Mark these cells with a cross as these cells will not need further inspection.
  For scouting purposes this may be all the information that is required.
  
• The eggs that have been parasitised will go a coal black colour in about 4 to 5 days.
  
• Some (say 5%) of unparasitied eggs may be unviable and just collapse.
  
• All the cells can now be inspected with a hand lens. Record whether eggs hatched (H), unhatched (U), or parasitised (P). Percentage parasitism can then be calculated.
  Unhatched or unviable eggs are not included in the percentage parasitism calculation.
  
• Wasps will emerge from black eggs in about 10 days after collection, depending on temperature. Up to six wasps may emerge from one Helicoverpa egg.
  
• Percentage parasitism checks will usually under estimate the actual level of parasitism.
  It is a question of "by how much"? This will depend on how "brown" or developed the eggs are when you collect them.
  Off-white eggs may still be acceptable to wasps when collected and may be parasitised if left in the field for another day.
  

Larvae counts
If larvae counts are above your threshold for that crop stage then spraying needs to be considered. If larvae counts are below the threshold then current and last weeks percent parasitism info will provide useful information towards the next spray decision (especially in crops where there is a tendency to spray on egg pressure rather than actual larval pressure).

When egg pressure is high there is a tendancy to spray on egg pressure alone. There are however various indicators that can provide valuable information especially when spray decisions cover large areas of crop. In particular, a little more time spent on sampling for egg parasitism can save many hours of spraying and thousands of dollars.

How many checks to do?
In crops where heliothis is an issue for large sections of the life of the crop (e.g cotton, tomatoes) then regular checks are recommended. In this way parasitism rates are factored into spray decisons and impacts of chemical applications and the weather can be seen.

In crops where heliothis has a narrower impact period (e.g sweet corn) its possible to do one parasitism check just before tasselling. The results will indicate what the likely level of parasitism will be in the following week. As a rough guide once you have 2 parasitised eggs per plant you can expect very high levels of parasitism in the following week even if the egg pressure jumps condsiderably.

So you've done your para check and you have the results. So what now?

The first question you need to ask is:
"How brown were the eggs when collected?" The more brown ring stage eggs collected the closer to the actual level of parasitism. White eggs have had less time to be parasitised by a wasp so this may underestimate the actual parasitism level. White eggs can be put in a separate row in the tray when collecting.

Percent parasitism data on its own can be misleading.
It needs to be combined with egg pressure counts to be really useful.

By multiplying the number of eggs per meter by the percent of hatched eggs you get the number of viable eggs per meter (or potential larvae per meter).

Likewise, by multiplying the number of eggs per meter by the percent of parasitised eggs you get the number of parasitised eggs per meter. More on this later.

When egg pressure is low (say <2/m) and parasitism is high (over 90%) then hatched eggs per meter is likely to be well below your spray threshold and the decision not to spray is an easy one.

As the egg pressure increases the decision making becomes more difficult. For example, say 50 eggs per meter with 90% parasitism. This leaves 5 viable eggs per meter. But before you assume that spraying is necessary there are a number of factors to consider. Firstly, how brown the eggs were at collection? If some of the eggs collected were white then the actual parasitism may be closer to 100%. Then consider recent history and current information e.g. last weeks para checks, percent para of white eggs, predator counts and weather info. See also Environment for more on weather.

Last weeks results
Recent results from parasitism checks and observations of the numbers of larvae getting through adds to ones confidence in making decisons for this week. For instance, the para checks may be steady on 80-90% but the last weeks observation of larvae numbers indicates that most of the 10-20% unparasitised eggs are not developing into damaging larvae.

The numbers of parasitised eggs per meter in the previous weeks results will also give an indication of the movement in wasps numbers. A high level of para last week of moderate to high pressure of eggs will yield more wasps this week. Conversely low egg pressure last week means lower numbers of wasps this week.

Sampling white eggs can be useful. High levels of parasitism in white eggs indicates that egg pressure could increase considerably without a fall in parasitism rates. It indicates that trichogramma are getting the eggs soon after they are laid and have to compete with other wasps for those eggs. Put white eggs in a separate row when collecting.

Other mortality factors
Larvae that hatch from unparasitised eggs may be killed by other means factors - for example, predators, larval parasitoids, pathogens and weather conditions.

Predator counts
Predatory beetles and predatory bugs can have a useful impact on larvae (and eggs). Predator counts of 5 or more per meter are likely to have a significant impact but precisely how much is difficult to assess with standard sampling.

The likely impact of predation is more difficult to evaluate than parasitism. In crops, monitored twice per week, that have high predator counts, the incidence of "brown" eggs is usually lower than would be expected from previous counts of white eggs.

Likewise, if egg pressure is high and parasitism rates are high then "black" eggs should be pretty easy to find unless predation levels are also high.

Conversely, high numbers of black eggs in the crop suggests low levels of predation as these eggs have been in the field for over 5 days.

Pathogens
Natural or introduced pathogens can have considerable impact. Previously applied sprays of NPV (Gemstar®, Vivus®) can have a carry over effect especially if there were good numbers of larvae infected.

The point is, as much as possible, spray on the numbers of larvae actually observed not on egg counts alone or even on parasitism checks alone.

Parasitised eggs per meter
Its worth being aware of the number of parasitised eggs per meter (eggs per meter x % parasitism from collected samples). If this is low, say 1 or less, then the wasps can be set back by hazardous sprays or unfavourable weather (extreme heat or several days of rain). If parasitised eggs per meter is 5 or more then recovery will be much quicker from adverse events.

If it rains for a couple of days this will inhibit adult trichogramma and if rain is heavy many will be killed. If egg pressure was high just before the rain started then a portion of these eggs will not be parasitised. So be prepared to intervene with a compatible larvicide when the rain stops.

Monitoring is often described as the cornerstone of IPM. This is because accurate information is the basis for appropriate decision making. However, monitoring that only considers pests tends to result in pesticide-orientated decisions. For example, spray thresholds based on egg pressure only, do not take into account egg mortality due to parasitoids and predators that can substantially reduce egg hatch.

In sweet corn, the intensity of the sampling process will vary greatly depending on the district, the amount of damage tolerable and the importance of natural enemies in that area. Sampling four consecutive plants at five sites within the crop is considered the absolute minimum. The larger the number of sites visited the more chance of picking up low level damage, emerging pests and natural enemies.

Monitoring record sheets can be devised for local conditions and should include the major and secondary pests as well as the major natural enemies usually encountered in the district. Numbers of the various life stages of heliothis should be noted: white eggs, brown eggs, very small larvae, small larvae, medium larvae and large larvae.

In some areas, egg parasitism of heliothis can be very significant and provision should be made to note "black" eggs. In these areas, it is also recommended to collect, just before tassel, "brown ring stage" eggs and grow them out to determine parasitism levels. See above.

Interpreting the data and deciding what action to take, if any, involves assessing the many factors operating in and out of the crop. These include: the stage of the crop, pest pressure and life stage, activity of natural enemies, effectiveness of various control measures, the weather, the value of the crop and the market tolerance to the various types of insect damage. Some in-field considerations are noted below:

Damage from heliothis can be higher than anticipated from the egg counts on a particular day. For example, if sampling shows 1 white egg per 20 plants, then over a week this may accumulate to 7 eggs per 20 plants. In the absence of natural enemies or chemical controls this may cause significant damage.

The effects of NPV (Gemstar®) may not be seen for up to a week. This can be disconcerting. Infected larvae can burrow into silks but usually exit to die before they reach the cob tip.

Secondary pest damage can be significant and should not be overlooked or confused with heliothis damage as control measures may differ.

Generally, few heliothis eggs are laid after the 7th day of silking. If natural enemies are present, the usually low number of eggs laid after early silk are more likely to be parasitised or consumed by predators.

Predation is more difficult to evaluate than parasitism. In crops (monitored twice per week) that have high predator counts the incidence of "brown" eggs is usually lower than would be expected from previous counts of white eggs.

Likewise, if high levels of parasitism are found in collected eggs but few black eggs are noted in the next weeks field sampling this also suggests a high level of predation.

Conversely, high numbers of black eggs in the crop suggests low levels of predation as these eggs have been in the field for over 5 days.