Investigating the potential use of Trichogramma, a hymenopteran egg parasitoid, in the integrated management of lepidopteran pests of cabbage in Puerto Rico

Investigating the potential use of Trichogramma, a hymenopteran egg parasitoid, in the integrated management of lepidopteran pests of cabbage in Puerto Rico

Summary

All research has now been completed in Puerto Rico and the student coordinator is back in Gainesville, Florida, writing up the thesis. Laboratory experiments confirmed earlier field experiment findings that Trichogramma pretiosum prefers diamondback moth (DBM) eggs as hosts over the eggs of the soybean looper. Furthermore two other species of trichogramma (Trichogramma mintum and Trichogrammatoidea bactrae) showed this same preference in the laboratory experiments.

Farming systems analysis of small-scale farms in the central mountains of Puerto Rico is being completed. Preliminary analysis shows that labor and market constraints have the greatest impact on farm activity composition.

Objectives/Performance Targets

• Investigate the parasitism of diamondback moth and soybean looper by trichogramma in cabbage production in Puerto Rico. Design of laboratory and field experiments will focus on the effects of the two-host system and of plant architectural development on trichogramma parasitism of each host species.

  Characterize the local farming systems and vegetable-growing practices of the central mountain region of Puerto Rico, using linear programming, with special focus on cabbage production and the integration of trichogramma into IPM practices.

Accomplishments/Milestones

All field experiments were completed in 2002 and extensive data was collected on the location of the two host species' eggs on cabbage. The work indicated distinct differences in positioning of the two hosts' eggs on the plant. Position and grouping of the eggs largely depended on the individual host species and on the architectural development of the cabbage plant with age. Although statistical analysis needs to be completed, it seems that the patterns and overall levels of trichogramma parasitism are influenced by the shifting positions of host eggs on the cabbage plant.

Any expression of host preference shown by trichogramma seems to derive from two sources. The first source is the intrinsic preference the wasp may have for one host egg type over the other. The second source seems to arise from the relative locations of the two hosts' eggs on the plant. Most of the parasitism by trichogramma occurred in the lower reaches of the cabbage plant. From observation, the trichogramma in the experiments tended to walk to the plants and begin searching from the bottom up. Either the trichogramma, after a certain time, made a decision to stop parasitizing and so stopped halfway up the plant or for some other reason, made the decision to leave the plant altogether before reaching the top. Whatever the reason, those host eggs found in the lower reaches of the plant showed higher levels of parasitism. Specific parasitism levels in each host species depended in part on where their eggs were found.

Aside from the influence of egg location, there did seem to be an intrinsic preference expressed by the particular trichogramma species used (i.e. Trichogramma pretiosum) for diamondback moth eggs. Laboratory experiments were conducted to examine this preference.

In the laboratory experiments T. pretiosum expressed a clear preference for DBM eggs over soybean looper eggs. This was also the case when the female trichogramma were exposed to one or the other of the host eggs for four hours before being released into the experimental petri-dishes. This 'conditioning' did not seem to influence trichogramma's preference for DBM eggs. Two other trichogramma species (T. minutum and T. bactrae) were also examined and they also showed a preference for DBM eggs. T. bactrae although preferring the DBM eggs, showed relatively high parasitism levels of soybean looper eggs.

Over fifty interviews with farmers in the central mountain region of Puerto Rico were conducted. The information from these interviews is presently being collated to use in the construction of a linear programming model. This in turn will be used to examine the dynamics of the farming systems found in this region of Puerto Rico. The main task of this model will be to examine the likelihood of adoption of IPM strategies on these mountain farms. Specifically we would like to see whether a diamondback moth management scheme, using trichogramma and a B.t. insecticide in cabbage is likely to be adopted by the farmers. To facilitate this an on-farm trial was conducted in collaboration with a farmer in Orocovis, Sr. Ruben Ortiz Ortiz. In this trial, the farmer managed the cabbage crop, from seedbed preparation through to final harvest, as he would have any other. That us apart from the control regime used against diamondback moth. For this we implemented a scouting regime and a weekly release of T. pretiosum, which was supplemented by a weekly spray of a B.t. insecticide. During the trial Sr. Ortiz kept a diary of exactly what he did, when he did it and how much anything cost. This diary gave a good idea of what was involved in growing cabbage with this particular crop protection strategy.

The idea is that the information from this on-farm trial will be interposed with the regular farming activities into the linear programming model to see how cabbage grown in this way fares as a viable alternative to other more conventional income-generating activities.

From the time spent in this region of Puerto Rico, it seems that there are many constraints to farming in this region. The small mountain farms of Puerto Rico are unique, as are any other farming systems in other parts of the world. ~70% of the farms in central mountains of Puerto Rico are 20 acres or less in size. They struggle to compete with the large farms of the southern coasts or with the cheap imports entering the country from overseas. All crops aside from plantain, coffee and banana are grown in plots no bigger than 5-10 acres and the produce is sold to intermediaries who, once they have a sufficient quantity of the product, then sell it on to supermarkets. Thus supplies from the mountains are not substantial or without complexity. The supermarkets of the metropolitan area prefer the steadier supplies from Canada, northern USA or the south coasts of Puerto Rico.

Aside from market difficulties, there are problems in obtaining sufficient labor for the mountain farms. The industrial developments on the island during the last century led to the disappearance of large numbers of agricultural workers. The introduction of the federal food stamps program in the 1970s also had a negative effect on the availability of labor for the local farms. A third influence on agriculture in Puerto Rico was the push to modernize agriculture and to develop more intensive cultivation techniques in the seventies. This led to sophisticated incentive schemes that have had an effect of promoting some crops over others and of promoting the use of agrochemicals. All these events have had an effect on what an individual farmer is capable of doing on his/her farm and on how he/she tries to accomplish his/her objectives. This will hopefully be reflected in the linear programming model that arises from this work.

Impacts and Contributions/Outcomes

The diamondback moth is a severe pest of cruciferous crops throughout the world, including the Southern Region. In response to reduced pesticide efficacy, due to insecticide resistance, we are investigating the use of natural enemies, such as the generalist egg parasitoids, trichogramma, that have the ability to prevent early feeding by DBM larvae. This research is complimentary to research on other DBM parasitoids in the Southern Region. The farming systems analysis (linear programming) portion of the project will help determine the effects of constraints, farmer objectives, and resources of the cabbage-growing farms of Puerto Rico on the successful introduction and sustainability of IPM strategies, such as the use of trichogramma. The results of this project will contribute to the acceptance of biological control as a management tool for lepidopterous pests of cabbage on small-scale farms. This in turn, will contribute to a reduction in the use of insecticides and an increase in the sustainability of cabbage production in the Southern Region.