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Crops > Vegetables > Crucifers > The role of spiders as predators
March 2016. A discussion that started out on the role of spiders against pests of cabbages quickly turned into the merits of the crop. Some members considered that the head cabbages were nutritionally poor and more emphasis should be given to local “cabbages” although quite what was not stated. The value of Bt rather than other synthetic insecticides was stated, however.
Abstract Spiders are thought to play a significant role in limiting pest outbreaks in agroecosystems such as vineyards, orchards and cotton. The diversity and impact of spiders in vegetable crops are less well understood, although there is evidence that predators may be important for suppression of lepidopteran pests in Brassica crops, particularly early in the season before parasitoids become established. Sampling was conducted in early season plantings of Brassicas in the Lockyer Valley (South East Queensland, Australia) in order to determine the most commonly occurring spider families. The most numerous were Theridiidae, which were more strongly associated with cauliflower and poorly associated with cabbage. The Lycosidae and Clubionidae/Miturgidae (formerly in the ‘catch-all’ family Clubionidae) also occurred commonly. Lycosidae (and to a lesser extent Salticidae) had above average abundance in Chinese cabbage and below average abundance in broccoli compared with average abundance for these spider families; Clubionidae/Miturgidae had above average abundance in cauliflower. Laboratory studies were then conducted to explore the predatory capacity of these three most commonly occurring spider families. All three were capable of feeding on larvae of the diamondback moth, Plutella xylostella (Linnaeus), and cabbage cluster caterpillar, Crocidolomia pavonana (Fabricius), under laboratory conditions. Theridiidae, which are thought to prey on small pests such as leafhoppers and aphids, were able to successfully attack larvae up to five times their body size. Predation rates varied from an average of 1.7 (SE = 0.47) (1.6 control corrected) larvae consumed over a 24 h period in the case of the Theridiidae, to 3.3 (SE = 0.60) larvae for the Clubionidae/Miturgidae.
The nutritional value of European (head) cabbage was made
Re the nutrient value of European Cabbage, this was a point made 60 years ago by the late nutritionist Lucy Hamilton(-Reid) in her pioneering work in Papua New Guinea: Hamilton, Lucy. 1955. “Indigenous versus introduced vegetables in the village dietary”. The Papua and New Guinea Agricultural Journal, 10(2), 54-57.
It was underlined more recently by many publications, including :
Bill Clarke had interesting observations on the role of European cabbage (relatively frost resistant, but poor nutrients) in the high altitude (>2400 m) region of the Marient Basin (Enga Province) in Papua New Guinea during and after the 1972 El Nino frosts. During the recovery, apparently much was grown for sale rather than eating. (see Clarke, W.C. 1989. “The Marient Basin, 1972-76: Recovery or adaptation?” Mountain Research and Development, 9(3), 235-247.
One member thought that the best crop for cyclone recovery was sweetpotato and not cabbage. Another wrote about his experience in Fiji and elsewhere on DBM. It is provided as sent:
IPM for head cabbage has failed in Fiji at least three or maybe even four times! BUT I believe there is hope. European cabbage certainly has its issues with DBM and also other pests in the Pacific and SE Asia. I agree with you about growing other crucifer crops as well (leafy brassicas can be grown okay in the wet and/or hot season). I am finding head cabbage somewhat easy to grow in Fiji and SE Asia at the start of the dry season = cool (pest pressure at its lowest). But a glut can then occur in the market, bringing about lower prices.
We had good success in Fiji 2 years ago (FAO project) growing head cabbage using crop scouting, finger and thumb for squashing pests in small crop areas, an action threshold of 10% of plants infested with medium to large-sized caterpillars (not spraying for small caterpillars). We let spiders and fingers do the job (or spot sprayed the plants). Also, Nitesh Nand (SPC Entomology technician) and others recorded up to 80% parasitism of DBM.
Spiders are very important at transplant and establishment stage. We are also finding (in NW Cambodia – NZAID project) that spiders are doing a great job on new transplants in predating on most foliage dwelling pests on most vegetable crops where we are using plastic mulch and drip irrigation.
Fingers/squashing pests and spot spraying are also important for Crocidolomia, Spodoptera and Hellula on brassicas. Flea beetles are an issue on small plants as well.
As you say, DBM is dangerous because of 1) insecticide resistance and 2) its habit of moving into the centre and eating out the new small (but crucial) growing tip of heading cabbage. We find spot spraying the growing tip for DBM and also spot spraying for clusters of caterpillars can be effective.
As for resistance, there are now enough different mode of action (IRAC MOA classes) insecticides to rotate when action thresholds are exceeded. We recommend starting with Bt on small plants and then rotate these different modes of action (MOA): chlorantraniliprole, indoxacarb, spinosad, abamectin benzoate and even fipronil. Some of these are not available or local DBM are resistant to them. BUT, if you rotate different MOA over time and do not spray too often then you can be successful, at least in the dry season.
Our other strong recommendation is to rotate different crop families – do not follow a brassica with another brassica crop in the same area.
Also, do not use the broad-spectrum insecticides (SP & OP) because 1) DBM resistant to them , and 2) you kill the important natural enemies. The more I walk crops in NW Cambodia, Fiji and NZ, the more I think spiders are so important on transplanted crops. Also, they do not need seeding/conserving into a new cropping area because they can quickly move into these crops (overnight).
In reply to a question about intercropping with root crops, yam, taro and cassava, and using Chinese cabbage rather than head cabbage, a member said the following:.
Yes, you can try inter-cropping, trap cropping and border cropping, etc. The main thing with DBM and brassica pests is to have a period free from brassicas to get rid of local populations, including resistant local populations. DBM only survives on brassicas.
So, mainly don’t keep on planting brassicas all the time.
In NW Cambodia we are using rotation over time as our main pest and disease management strategy. That means rotating different crop families and also rotating available MOA insecticide (when scouting leads to a spray decision).
I have not done research with Bele, but we are controlling stem and fruit borers with rotation of a range of different MOA insecticides. The main ones used in SE Asia seem to be emamection benzoate and chlorantraniliprole? So, scout to assess when and if small larvae of Aibika tip borer is appearing, and apply only insecticides that target boring lepidopteran larvae (caterpillars that bore into vegetables). These include insecticides with some systemic activity, so need to assess withholding periods (WHP). The MOA insecticide classes available in SE Asia include IRAC classes 28, 6, 4, 22 and 2. I don’t know what is available but in general: 1) scout twice weekly 2) only apply insecticide to perceived need 3) use selective insecticides if available 4) consider withholding periods (WHP).