Entomology | [Entomologia]

Emphasis in efforts to develop host plant resistance to spittlebugs (Homoptera: Cercopidae) in Brachiaria grasses has shifted from germplasm introduction to improvement of key accessions through breeding. Consequently, the Entomology Section is developing an infestation method to assure adequate selection pressure for field evaluation of progeny of sexual crosses. In addition, a bioassay system has been developed to screen chemically defined plant fractions and individual compounds for biological activity when ingested by spittlebug nymphs. No outstanding Brachiaria accessions have been identified from collections established in Carimagua during 1988 and 1989. The best adapted accessions were heavily attacked by spittlebug while those accessions that had low infestations were poorly established or failed to complete coverage of the test plots. Progress has been made on development of a spittlebug bioassay to test chemically defined fractions of resistant and susceptible Brachiaria accessions. It should be possible to test the hypothesis that antibiotic resistance in some Brachiaria accessions is due to compounds present in the xylem, site of spittlebug feeding. Extracts will be tested to identify active compounds responsible for high levels of antibiotic resistance in B. brizantha cv Marandu, B. jubata CIAT 16531, and B. jubata CIAT 16203. The bioassay consists of growing susceptible plants (B. ruziziensis CIAT 675) in test tubes using a specially designed feeding chamber. Plant roots grow the length of the tube passing through a chamber where spittlebug nymphs are placed, and into a nutrient solution that contains the plant fraction or chemical tested. In a preliminary trial, response was obtained to 20-hydroxyect-dysone, the insect moulting hormone. Testing of plant fractions provided through collaboration with Cornell University has begun. Lyophilized spittlebug excreta obtained from adults fed on different Brachiaria hosts were examined by HPLC chromatography. In samples from adults fed on the highly resistant B. jubata CIAT 16531, peaks appeared that had no corresponding peak in excreta from adults fed on B. decumbens cv.Basilisk, indicating the presence of compounds unique to B. jubata CIAT 16531. Future analyses will include plant xylem sap in attempt to identify unique compounds present resistant accessions. Extracts will then be tested by the bioassay describe above. An economic threshold has been calculated for establishment of A. gayanus in areas of savanna infested by the leaf-cutter ant Acromyrmex landolti. Based on estimates of colony density in native savanna and knowledge of the relationship between colony density and damage to germinating seedlings, loss at establishment of A. gayanus may be avoided. There is an exponential relationship between colony density and establishment success. Based on this relationship, growers have the option of controlling colonies chemically in fields with infestations higher than a threshold establishment level or they may decide to plant a more resistant variety such a B. humidicola or B. decumbens. To enable growers to accurately estimate colony densities in areas to be planted, a rapid sampling technique has been developed as a basis or the selection of appropriate germplasm or control action. Based on extensive mapping of ant colonies in native savanna at Carimagua, a computer generated map and sampling procedure determined optimal quadrate size and orientation to minimize variance in estimates of colony density without increasing the effort involved in sampling. In addition, cost of chemical control of colonies was calculated. With information on threshold damage levels, sampling methods, and cost of control, growers will be able to decide how best to deploy available germplasm options according to pest density before planting.